I see 0 difference between using the two or any actual use case

https://stackoverflow.com/a/31629588/1118719

https://webmasters.stackexchange.com/questions/96955/speed-comparison-absolute-vs-relative-path-links etc.

1st one says absolute may be better if using a CDN which both Wikimedia and Miraheze do but is talking about loading resources.

2nd one refers to the slight changes in page size + rendering time by your browser which aren't going to be noticeable when using href links.

(My guess is the authors of MediaWiki made the conscious decision to use relative links.
But some extension authors didn't know how to make them, so just used absolute links.

Just like we know when editing Wikipedia to use [[Football]] instead of [https://en.wiki..../Football Football].)

https://github.com/libwww-perl/WWW-Mechanize/blob/master/script/mech-dump

Note
$ lynx -dump -listonly https://...
converts all links to absolute, so is useless for debugging this issue.

Indeed you can simply do:
$ GET https://... > /tmp/e
Then:

$ perl -nwle 'print for /href=............./g' /tmp/e|sort|uniq -c|sort -nr
     54 href="https://radi
     19 href="/wiki/%E7%89
     19 href="/wiki/%E4%BD
     11 href="javascript:v
     10 href="/w/index.php
      4 href="https://meta
      4 href="https://crea
      2 href="/wiki/%E9%A6
      1 href=\"https://cre
      1 href='https://radi
      1 href="https://www.
      1 href="/w/opensearc
      1 href="/w/load.php?
      1 href="/favicon.ico
      1 href="/apple-touch
      1 href="//login.mira
      1 href="#searchInput
      1 href="#mw-head"

So we observe 54 + 1 = 55 external links that should have been just local links.