The Swiss Pass is valid till Domodossola, so you would normally buy a ticket Domodossola - Venizia, and you could buy this with Trenitalia. You would then just board the EC you booked with Trenitalia already in Lausanne. Your STP covers you.
This summer is different, because there are works along the route, and so it is partly closed. There are busses between Domodossola and Milano, and between Stresa and Arona. To make it a bit less incovnenient for the passengers there are also a couple of direct Lausanne - Milano busses. That is the bus you are seeing.
To book this just select “GA” under reductions, and the system will know that you have free travel in Switzerland.
