I've known multiple with glucose monitors and all of them told me they saw no increase after drink a diet soda. I think the "aspartame causes sugar spikes" is an urban legend. I never get the sugar rush from them like I do if I drink a sugary soda.
IANAD, but my reading of Dr. Jason Fung's book "The Obesity Code" suggests that while artificial sweeteners do not spike glucose immediately, they elicit an insulin response, which over time causes insulin resistance, which over time increases glucose.
In the short term, but as I understand, the constant insulin response would also cause insulin resistance over time.
But I understood parent as doing immediate measurements and expecting blood sugar increase despite insulin produced with no carb intake.