Can you "invent" a definition?
https://en.wikipedia.org/wiki/Physical_constant#Table_of_phy...
Archimedes, Pythagoras and Euler might agree.
At least Newton's lab of universal gravitation and Coulomb's law defined their eponymous constants directly.
In other cases the people doing the defining named them retrospectively after the originator of, or most significant contributor to, the concept that motivated the definition.
The point is that we ascribe enough invention to these definitions to commonly name them after the people who invented them.
It is fascinating to dig into definitions and conventions that we know, and to realize that much of what we think of as universal fact is actually an arbitrary choice of history, canonized by time.
It happens often in math that people decide on arbitrary definitions. One fun example is Donald Knuth was fond of defining 0^0 = 1, while some other mathematicians prefer to define 0^0 = 0.
Next they'll invent the 'Ocu-ternion', which is the same as a normal Quaternion but each scalar value is divided by a constant because...VR? Facebook? Who cares, the point is they own it.
if they try to claim patent or trademark that's obviously a problem, but otherwise is this so terrible?
The header file they provided is BSD licensed. You can use it freely, forever, for anything you want.
(EDIT: Obviously, this doesn't invalidate your conclusion, but your conclusion doesn't follow from the premise, at least.)
I don't love Facebook's products, but I struggle to see the downside in them publishing this idea.