Agreed, though I maintain that it's in general unclear exactly which properties the advance width is constant with respect to. (For tabular numerals, is-digit-zero-versus-one clearly is, but what about digit-versus-question-mark when writing "27'3?? items" to indicate three significant figures? What about "+" and "-", which should have the same width as each other, despite not being numerals at all?
Full monospacing solves this, but that's often a stronger constraint that you actually need.)
Also, nominally monospace fonts often end up having character-dependant width anyway (ranging from blatant bugs like substiting fallback characters without forcing them to the correct size, to completely legitimate-ish variations like CJK ideographs with integer multiples of the normal character width). So, I'd still say tabular numerals are more like a weaker form of monospace than a stronger form of uniwidth - constant width across styles may or may not be relevant, but if it is, it's table stakes (no pun intended).