The answers to this question should not focus on how/whether this
organism could survive (sources of food), travel or even evolve.
Well, I was educated as a biologist so it's gibberish to me to think of a model for animal that doesn't start with its environment and its selection pressures.
So, I am going to ignore that restraint in part because doing so makes it easy to imagine why a space turtle would be non-spherical.
1) The turtles aren't spherical because they face selection to form another shape.
Spheres don't make good powered shapes. A lot of people think that real spacecraft remain cylinders because that form is needed for launching through and atmosphere but it's really about stability under power in space.
Stability is a function of the displacement of mass laterally from the line of thrust running up from the engines. The closer the majority of mass is to the line of thrust, the easier to balance the ship on top of the thrust. This is not obvious because in air, water or ground contact, a long cylinder shape encounters resistance and begins to act like a lever knocking the ship off the thrust axis. In vacuum that effect is trivial. The ship's axial center of gravity is more stable.
A sphere displaces more mass around from the thrust line than a long cylinder of equivalent volume. That displaced mass must balance constantly or it will move the ship's center of off the thrust line.
Consider it this way: Inside a ship under acceleration, an astronaunt walks as far along the longest path possible. As he does, his mass alters to the center of gravity to some degree. In a cylinder ship, he will spend most of his time slightly offset from the thrust line and moving parallel to it. On a spherical ship, most of the paths he could take move him farther away from the center of gravity and in the most extreme case, as long as half the diameter of the ship. If he walked side to side at the ship's midpoint perpendicular to the line of thrust, his mass would have a lever effect at the extremities.
So, spheres are great if you have no thrust but otherwise tricky. Just as both animals and vehicles on Earth have the basic head-tail layout despite their radically different origins and materials, likely a space animal would also evolve have a head-tail layout for stability under thrust.
The next issue would be wider than it is thick. Animals on earth evolved under gravity so they form to resist gravity which means they have a top-bottom. A space turtle wouldn't. If it evolved as a head-tail form as above, everything else would be distributed evenly around the outer perimeter.
To get a flat shape, we would need to evoke a selection pressure to make that shape optimal. The obvious one would be that the turtle absorbs sunlight or solar wind plasma for food. In all three cases, a flat shape oriented perpendicular to the line to the star, like a sunflower, would give it the most surface area for absorption.
2) Size: Just because something is big, doesn't mean it has to be a sphere. Gravity is the weakest force so if offset by another force, its tendency to form materials into spheres could be easily offset.
Living things are not static structures, they are dynamic and exert energy all the time to maintain their shape (cells devote 70% of their energy moving around potassium and sodium ions that among other things maintain cell wall shape.)
If we imagine a turtle that produces and controls magnetic fields in its tissues, quite plausible with biological conductors, we could imagine a turtle that constantly manipulates a complex matrix of internal magnetic fields to maintain its shape against gravity and to dampen out the various stress forces propagating throughout it. The magnetic fields would actually be the most rigid part of the system. Instead of trying to simply resist giant scale stress forces with static materials, the turtle would absorb, diffuse and redirect them.
The turtle would be something like an internally complex balloon animal with gravity taking the place of external air pressure and magnetic reinforcement taking the place of internal air pressure. The tissues are the skins of the balloons. The tissues just have to be strong enough to exist at the balance point of each force. The forces actually strengthen the tissues just like air pressure strengthens the skin of balloons.
In that case, the turtle could grow much larger than it would if it relied on just static mass to resist the pull of gravity. However a consequence of dynamic form would mean that as soon as the turtle dies, or just weakens past a certain point and runs out enough energy, it will implode.
Also, growing in a rectilinear turtle shape would distribute mass around in a non-spherical shape which would decrease the overall power of gravity to pull everything to a single point. This would slow the gravitational feedback loop that forms spheres.
I presume at some point gravity would defeat magnetic reinforcement but I don't have any idea how to calculate the mass at which that happens, especially with an irregular shape.