OK, I stand corrected. I was looking at the Bahn website, which, anytime the 840 route between Berchtesgaden and Salzburg is shown, says that the Bayern-Ticket is a partial fare. But that's not the first time I have found the Bahn website to be in error when a bus use is included. Funny, the Bahn website included the bus from Berchtesgaden Hbf to Königssee with a B-T starting in Munich (M to Freilassing, BLB train to Berchtesgaden), but not the bus from Berchtesgaden Hbf to Salzburg or even to Marktschnellenberg (the last stop in German on 840) with the same route to Berchtesgaden.
As for the contradictions identified by SLA, I think I would always refer to the AGB as the final word.
if you want to buy a Bayern Ticket and ride the 840 bus to B'gaden and the Königssee, then return
in the evening by train to Salzburg Hbf, as Lee has suggested, it looks to me like that is possible.
Possible but not practical, at least for less than four adults. The BGL TagesTicket Bus & Bahn, at 12€ per adult, will cost less than the Bayern-Ticket for three or fewer adult users.
There are also a few other differences for the BGL ticket vs. the Bayern-Ticket. The BGL ticket does not have the 9 AM restriction, but it doesn't say it is valid until 3 AM the next day, either (kind of moot since the train stops running before midnight). The child provision is slightly different. The BGL ticket also gives you discounts on, for instance, the bus up the private road to the Eagles Nest, the Königssee boats, and the Salt Mine and Therme in Berchtesgaden.
Note: there might also be discounts with the Bayern-Ticket. There used to be a listing online of discounts available with the Ticket, but I can no longer find anything.