I'm not up on seal anatomy; but I think the main thing to evolve is some sort of grip on, for example, a stick or spear.
That does NOT require an opposable thumb. Our opposable thumb gives our grip strength, but I have known a man that lost his right thumb in an accident (stuck it into an exposed boat propeller, had it surgically amputated). Using four fingers he could still grip and hold things and write with a pen: He held the top part between his index and middle finger, and near the tip of the pen between his middle and ring finger. So the middle finger wrapped over the pen.
Just gripping a stick, one can break off branches to form a shaft, and rub it against rocks to sand one end down into a sharp point. Likewise, one can form thicker shafts used as levers, and sand flattened ends to use as shovels. Or, as paleo people seem to have done, attach handles to the shoulder blades of animals to make them shovels.
A grip likewise allows the winding of cordage, the application of natural tars or pitch. And you don't need an opposable thumb to hold a rock and knock chips off it, by striking it against a larger rock on the ground. It is possible that using their lips, seals could tie knots as well. And don't forget, if the seal can grip a handle, it can have fine-pointed and sharp tools for details with large handles on them; such as drills, or picks. In that way they can also finely carve bone to make tools.
It is an extremely anthropocentric and misguided view to think opposable thumbs are the be-all and end-all of tool production; it only shows a lack of imagination. Even birds make tools with just their beak, and sometimes feet (which are adapted to grip branches (the precursor of sticks)).
Intelligence is the key, if the seals can grip a stick, and have human levels of imagination and problem solving, I'd expect them to develop a wide repertoire of tools, furniture and shelters. I'd expect them o master fire. It won't look like a tool for a human hand, of course. Those are just an accident of our own development, we made stuff easy for us to use. They will make stuff easy for them to use, using one flipper, two flippers, and perhaps their lips, teeth and jaw. Heck, we humans still use our teeth to get a grip on some things our fingers cannot grab.