The reason Artifex is able to license MuPDF in this way is because it owns all the copyright in the MuPDF code. The vast majority of development work on MuPDF has been directly paid for by Artifex. The rest of it is in smaller patches and fixes that have had their Copyright assigned to Artifex.
If Artifex were to take on a patch without owning the Copyright, it would no longer be able to license MuPDF commercially, which would mean that it could no longer fund development.
Accordingly, in order for any (non-trivial) patch to be accepted into the main MuPDF source tree, Artifex asks the author of that patch to sign a Copyright Assignment letter giving over their copyright to Artifex.
Artifex runs a Bug Bounty programme, whereby interested developers who tackle nominated problems can earn cash payments for their fixes.