OP isn't wrong. Once the switch was made from 2D to 3D gates, transistor numbering became "this is the performance you would expect if you COULD shrink a gate to x nm", and while it's inaccurate it lets people understand how differentiated a new generation is from the previous ones. It also lets them stay consistent with understanding the progression of chips over time as in most of the history of silicon the generations were defined by the gates' nm sizing.

I don't see it as an issue if the marketing term maintains the original context:

"how large a planar FET should be for the transistor density used in this process"

Is this the case, I'm not entirely sure. If it is, there be a better unit for this measurement, absolutely.

PLANETS: PLANar Equivalent Transistor Size

Or hire a marketing team to figure a good one.