 Did they back up all of their posts? Will they really go dig one out for you if you need to know the in depth conversation they were having about whatever the fuck I ended up on that page for?
I don't think this should be dunked on. This does drive some traffic and isn't a worthless action to do if you're deleting your reddit account anyway. Their choice being lemmyworld is shit but if people doing this results in just a handful of extra users created then it's better than not doing it.
I don't mind the advertising for lemmy, what I do mind is that this person thinks anyone is going to miss them off reddit and will come look for them, and they think anyone is going to reach out to them literally just to lookup whatever their post was. The gall to assume literally anyone cares about what they posted on reddit is hilarious to me.
A ton of tech support threads are now dead from deleted comments, so if you're active on those it makes sense that someone might want to know the solution to some computer issues that's now been replaced.