Posture is steady and comfortable.

Three words, and the entire classical definition of posture. Sthira is steady; sukha is easeful. Both, together. Not one bought with the other.

In the Sutras, āsana means the seat you take for breath work and meditation. A good test for your own sitting position: can you hold it still, and can you forget about it?

Quoted to justify every modern shape, however demanding. The verse is describing a seat you can stay in for a long time, not a pose you survive for five breaths.