I suspect that the team is not going to run a full proper 6 man rotation, but instead pull every lever they can to get as many starts as possible on 5 days rest. I think that distinction is a nuance that's hard to fit in a tweet, and frankly Levine's not a great writer at this point, hence him saying 6 man rotation
To your point the guys who are not slam dunk starters will end up filling in in a number of ways. I'd expect at least two guys on the staff on any given day to be A) not one of the traditional 5 SPs and B) capable of giving you multiple innings. Whether that's a sixth regular starter temporarily on the roster, a long reliever (able to go 3+ innings and face as many as 15ish batters) or let's call it a medium reliever (2 innings or 8-10 batters).
I know they had an extra roster spot to play with due to expanded rosters, but the September pitching staff is probably a good model. They had Civale in long relief, Soroka/Brown/Wicks in medium relief at various points of the month, and Assad making spot starts. That was in addition to five tradtional SPs and 6 traditional short relievers.