The theory is that since most of Venus’ atmosphere is CO2 at this level, the breathable atmosphere of a human habitat is actually bouyant, which would make suspending a colony much easier.
Doing something like that on the scale of a research presence like the ISS is within the realm of current technology-- but you are right that doing so for a whole city is not technically possible at the moment-- nor is true space colonization in general, I would argue. There’s a lot of unknowns and unsolved problems.