To clarify matters, keel material - wood in the age of sail, but metal later on - resists any movement as if by friction, but resists movement much less along one axis, and also resists somewhat less when turning. This resistance has nothing to do with air, it simply occurs relative to the largest object nearby, typically the planet.
If one was to push a length of keel, it would resist lateral movement, but would 'slide' forwards and backwards with only a little friction. If a length of keel was placed at an angle to the ground and pushed at one end, it would fall down because it could turn, though it would do so more slowly than expected if it was an object subject only to gravity.
However, if a length of keel was placed at an angle to the ground and held at that angle while being pushed, it would ascend at an angle somewhat less than the angle at which it was being held, depending upon how fast it was moving and its lateral friction coefficient. The higher the lateral friction coefficient, the less leeway a keel will make. This is all completely independent of the atmosphere, if any.
This means that as long as a vessel with keels is moving, it can maintain or increase its altitude. Because of the higher lateral friction, a vessel can be sail powered in an atmosphere - the keel gives the vessel something to push against so that there are differential forces between the wind and keel, allowing movement and steering in directions other than with the wind.
If a vessel with keels in gravity is overloaded or moves too slowly, the keels' angle of attack will be too great, resisting forward movement, and leeway will become greater than headway, resulting in loss of forward motion and a loss of altitude.
An object with multiple keels cannot be "anchored" in a gravity well: even the best keel material does not have infinite lateral friction, nor does any keel material have zero friction to headway. The best that can be expected is a greatly reduced rate of descent. In order to move, a keeled vessel requires an external source of kinetic energy - hence all such vessels require wind power, some other motive power, or gravic potential energy that can be traded for kinetic energy.
When we have magical keel material that allows flight with a smaller volume of material than an airfoil, when propellers can be made by placing angled keel material on the edge of a powered wheel, then only scarcity of that keel material might lead to the development of aerofoils, or perhaps they might be invented even if they are more expensive than keels, if they had some utility that is not served by keels.
For purposes of this question, let us assume that basic keels are a relatively rare form of wood, and are about as expensive as a rare hardwood such as Ebony. More efficient metallic keel material is possible in the age of steam.
Aside from this keel material, we may assume that the environment is much as Earth. Square sails which simply provide drag were invented around 3000 BC, and only allow sailing with the wind. Lateen sails were invented around 900 AD, and allow sailing into the wind, since they act as an aerofoil, but it was not until DaVinci first described man-made wings that their use as an aerofoil began to be recognized, and not until the 19th century that aerofoils began to be used for flight.
Additionally, keels operate by a different principle to aerofoils: aerofoils work by passing through a fluid medium, while keels resist movement in all directions, but offer much less resistance to movement along one axis, relative only to the keel and the rest of the universe.
For purposes of this question, let us assume that keels were discovered around 500 AD.
So, the question is: would aerofoils be invented earlier or later relative to other technologies in such a world, and to what uses might they be put?