Okay, after reading some source code I managed to get it to work by
searching for the query 'to:yoctocell@disroot.org git' and not just
'git'. But for some reason searching doesn't work with my other account,
which was why I had to specify the to address in the notmuch query.

I modified the `nnselec-compress-artlist' function and put a print
statement in it.

;;; Helper routines.
(defun nnselect-compress-artlist (artlist)
  "Compress ARTLIST."
  (let (selection)
    (pcase-dolist (`(,artgroup . ,arts)
                   (nnselect-categorize artlist 'nnselect-artitem-group))
      (let (list)
        (pcase-dolist (`(,rsv . ,articles)
                       (nnselect-categorize
                        arts 'nnselect-artitem-rsv 'nnselect-artitem-number))
	  (print articles) ; print here
          (push (cons rsv (gnus-compress-sequence (sort articles '<)))
                list))
        (push (cons artgroup list) selection)))
    selection))

When querying for 'to:yoctocell@disroot.org' it outputs a list of
arbitrary number, but for my other account it ouputs a list of nils
which is why the '< function gives an error.

It seems to be some misconfiguration on my end so I will see if I can
fix it.

Thanks for helping me debug the problem!

-- 
yoctocell
PGP fingerprint: 9DDD 8877 BC0C A9CE 5D8F  9ED6 0F9A 432E 305F DAFF