Not the GP, but... it has a specific meaning in the sciences:

https://en.m.wikipedia.org/wiki/Femto-

I'd imagine the argument against using it is that it's being used in a less precise way. Femtocell is a name commonly used for a consumer-level device to extend a cellular network. One could argue that use of femto should have some bearing on the radio being used.

I misunderstood, and assumed femto refered to the board size, as compared to the original limesdr.