They have more reasons than that, though.
A lot of artists don't want their stuff on e621 because they think that's where art "thieves" get stuff.
They think this because most social media accounts and subreddits dedicated almost entirely to reposting furry art source back almost always to the e621 link.
Which is stupid, because what actually happens is that they most likely found the stuff elsewhere and only linked e621 because that's the place where you're more likely to find info -like who the author is- on any furry picture. And because if the social media account of the artist is for whatever reason unavailable, people can still find the e621 post. And in the e621 post, they'll find the link to the real source anyway.
Another reason is that they don't want e621 to "steal" their precious clout (in the same way they think that posting the picture to r/yiff is art theft). Because they have completely confused social media high scores with actual career success. These people are also the ones who get pissy when someone more popular than them QRTs their post or get mad at other artists for "working for free" or "underpricing" their commissions; or who make transformative works of someone else's IP without permission, but then yell kys to people doing transformative work of their own fanart by using it for RPing or whatever (and then they still complain when hasbro or gamefreak DMCA's them).
Overall, it does seem that people on the DNP are just really fragile and egocentric and have fallen in love with the system that oppresses them.