It seemed to be an exaggeration in context, but he did support his claim, as the deaths of many Christians in churches on a Sunday made people reject spiritual explanations for the event and start seeking naturalistic ones.
I mean to send this to you when I saw the question but I was on my phone and forgot about it until, uh, now, so here you go...
I asked a similar question 6 years ago and got nice answers from /u/MrMedievalist and /u/textandtrowel.
This was a vague side research interest of mine a few years ago, and I noticed that while the strong link between the earthquake and the enlightenment is claimed from the people writing about the earthquake... it is not claimed very often by the people writing about the enlightenment. Also the traditional date pegging the start of the enlightenment is well before 1755. So make of that what you will. :)
Most modern studies of the earthquake focus on the fact that this earthquake has been mostly forgotten in European countries outside of Portugal, I think if you put your back into it you could argue it triggered an enlightenment movement in Portugal, but as for other places like France and Britain, the people there just weren't materially effected enough by it to care beyond abstract compassion. The earthquake was certainly an issue that was worked on in enlightenment circles, and analyzed by people, but as for it being the "cause," the timelines really just don't add up.
I recommend the 2015 pop history Gulf of Fire if you're interested - I read it and it was nice. Author discusses the enlightenment, and certainly like most historians of the earthquake thinks it was a big part of it, but he doesn't make a causal claim (if I remember right, since I read it 7 years ago now).