No, there's a very simple definition for these XXnm numbers and a very good reason why they only roughly correlate with transistor density.
These numbers refer to the length of the channel in the MOSFET or FinFET between the Source and Drain dopant regions. The channel is always the smallest feature the fabrication process can create, but how it's created is actually with lots of solid-state physics tricks (such as annealing to cause the dopants to spread out from where they were originally implanted, looking a lot like a physical Gaussian blur), as the original masks used to define the transistor are nowhere near that small.
Depending on how small they can get the mask details (mostly an issue of optics and mask creation, now bounded primarily by the frequency of light they expose to the mask material, though interesting holographic tricks can help if they've left the laboratory phase) you can define the minimum size of the transistors, themselves. But there are other considerations like heat dissipation and the melting points of the various metals you've used that may force the transistors to be larger than what you can theoretically build.
The channel length is still very important because it determines how many Coulombs of charge you need to switch the transistor on, and how long it takes to propagate a signal across the transistor.
It also correlates with how much leakage current the transistor has (smaller = higher) and therefore the standby power consumption, so you do continue to make an engineering trade-off to get lower, which is also why server hardware tends to get the smallest sizes first, as they least care about idle power consumption if they're being utilized correctly.
Everything I've said is about 4 years out of date, when I switched from EE back to Software Engineering for financial reasons (and also my wife is an EE so we didn't want to put all of our financial eggs in one basket), but the semiconductor industry moves so slow, and proof-of-concept fabrication techniques take so long to productionize, that I doubt this is very far off.
The only thing marketing has done here is fight between each other on whether or not channel size is the number to focus on, as it benefits their corporation or not.