It's the "momentum spot" because it was possessed, not because it was intercepted.
A distinction without a difference.
It was possessed because it was intercepted. If it hadn't been intercepted, it wouldn't be a "momentum spot", which, as you said, isn't even a defined term in the rule book, only a concept.
It's like saying you aren't bagging the end of a scrimmage kick, you're bagging the spot of possession by R. Yes, but that possession is only an issue because it's the end of a kick.
If the defense intercepts the ball inside the five and is moving backward, you had d@mn sure better have a bag on that spot!