I guess you don't get it. Tridium has their Niagra Framework. Other vendors don't make a box that communicates Niagra Framework. It's proprietary.
Further, when you are interfacing multiple protocols in the box, you are essentially having a gateway of sorts in the box itself.
This would differ from a 100% Lonworks network whereby the supervisory box(es) and the controllers can all be peer to peer under the same architecture. So everything is interchangeable. (I choose the word carefully)
Future conversion of Tridium to something else would be about as expensive as converting that Siemens to Tridium.