As gravity increases, buoyancy also increases. A high-g civilization with access to hydrogen would be able to float to the top of their atmosphere, and then proceed to launch into space.
Not a physicist but I thought the main problem is not getting high, but getting fast in the “horizontal” dimension to balance out high gravity and stay in orbit.
This really wouldn't help as much as you'd think. For a high-gravity planet, the atmosphere is mostly a trivial problem compared to gravity, and the gravity at the top of the atmosphere would be barely lower than it is on the surface.
A much more realistic and useful option is to just go air-breathing nuclear, and use the atmosphere for reaction mass.