You need to book with DB to have acess to DB lounges. It's stupid (and doesn't work that way anywhere else) but it is what it is. If lounge access is important to you, book with DB. If you're unhappy with the pre-assigned seats, you can always get a separate seat reservation from ÖBB for 3€ pp.

But personally I wouldn't go out of my way to get lounge access. You'd normally spend maybe 15 minutes at the station, and DB lounges aren't cool enough to make earlier arrival worth it.