Additionally:

When we were looking at the literature on web typography and accessibility, we read a study focussed on dyslexia that found that denser line heights lead to better comprehension.

The more cramped line height is a serious issue of accessibility.
People who have visual processing issues, people who have dyslexia, etc - they're finding it harder to read and enjoy Wikipedia (as you can see in the various reddit threads, Discords, and other comments).
I'm autistic, with processing issues. I spend a LOT of time editing Wikipedia, and improving articles. Right now, I can't. Right now, with this squished-together line height, I can't read it. And I can't *change* it.
I'm my everyday life I've done work teaching about how to make content accessible: font choice, and yes, font spacing choice, is a HUGE issue.
I highly recommend looking into and considering this as an issue of accessibility and even potentially ableism. The current changes have rendered the site far less usable for many Wikipedians, and the rationale is not sufficient.
Additionally, as a researcher myself, I do not feel that a study of 2,500 users is sufficient to make such a massive change, which so deeply affects the site's usability.
Keep in mind, as well, that many of the people who are describing being negatively impacted by this change are the same people who do a lot of editing, writing, maintenance, etc. Neurodivergent people love sharing information.
Look, I understand the rationale, but I do not feel it is justifiable.
At the very, VERY least, you should make it completely easy to opt out of.
I used to spend literal hours a day working on Wikipedia. Now, I can't. As a disabled person, it is no longer accessible. It's a shame.