@SleeplessOne1917 seems to be their GitHub handle. You can see this by going to one of the pull requests that are linked in the thread (e.g instance block feature)
@cannotsleep420 seems to be their lemmy handle. You can see this by going to the thread link the OP posted, in which that account is commenting about the UI tweaks.
Both accounts list their location as ‘Burgerland’(?) so they’re most likely the same dev. Plus the ‘SleeplessOne1917’ lemmy handle you linked has absolutely no activity.
You can also see @cannotsleep420crossposting earlier lemmy changelogs that mention @SleeplessOne1917…