What about tefilas haderech?

The gemara in Brachos on 46a going onto 46b says that a bracha hasmucha lachaverta is the only type of bracha that does not require a baruch at the beginning. What about tefilas haderech? It's not next to another bracha but it doesn't start with baruch! The Shulchan Aruch quotes the Maharam M'rutenberg who would say tefilas haderech after birchas hashachar if he was travelling in the morning. The Mishna Brura adds that if you're not going to say birchas hashachar then you can make a shehakol or something immediately before saying tefilas haderech (the fact that you take a sip is not considered a hefsek). He says that you don't have to do this but it's preferred. That doesn't really answer the question though. If the rule is that only a bracha hasmucha lachaverta does not require baruch then why were the rabonon mesaken tefilas haderech without baruch if it was niskana as a bracha bifnei atzma?

Rabeinu Yonah (earlier in the masechta) answers that any bracha that was made as a bracha hasmucha lachaverta does not require baruch even if it is said independently. This was really part of shomei tefila in shomone esrei.
The Tur answers that it wasn't really made as a bracha. It's supposed to be a tefila so it didn't start with baruch. However, he says because we talk about so much in the tefila, the rabonon decided to conclude the tefila with a baruch. It's still not a "real" bracha.