I agree that it’s not a blanket statement…waitlist use will likely vary by school.
Some schools released RD decisions before the pandemic became a ‘thing’, while others released after it became apparent that it would be bad. The schools in the former bucket are more likely to not have over accepted, while the second group may have increased their admits as you mentioned.
The schools that over admitted are also the ones that may be less likely to extend their May 1 deadline. I listened to a webinar where a UCLA director of admission said no way they will extend the deadline to June 1 because if they did they would be over enrolled.