SDL 1.2 could call GL just fine. And it does. No, the game was done like that on purpose, on being light and playable.

Any serious libre gamer with a big graphics card to test would just launch Flightgear at 4K with the ALS effects on, real time weather and a crazy draw distance over 30 kms with all the shading FX' on. And lots of tweaks that made clouds looks like clouds and not fancy textures or barely recreated shaders.