“The admittedly limited data ive seen (and anecdotally asking around) this does not seem to be the case”
Not sure where you’re asking around. Lakewood-type guys don’t get married at 18/19, they keep learning and get married in their mid twenties. I’ve never heard anyone deny this part of the observation.
“Did “closing the gap” help ? what has been accomplished?”
Closing the gap would help. It’s not something that needs to be tested. That’s like saying in order to accept that 5 – 2 = 3 you must have 5 objects and take away 2 otherwise there’s no way you could possibly know. If it hasn’t “helped” it’s because they failed to close it. I’ve seen no evidence that the Yeshivish tzibbur has changed anything in their marriage-age habits.
A partial gap-closing will not help; it will just stall the problem a little more. Even a 1 year age gap will cause surplus and eventually a “crisis.” In an exponentially growing population, eventually even the difference between one year and the next will be massive.