Danhiel These rules are valid only if you buy a through ticket from Spiez to Venice in one transaction.
That is no longer the case. Trenitalia does honor the connection even if the segments were booked independently. The thing that matters is that you do not plan connections below the minimum connection time. For Milano Centrale that is 15 minutes (and is quite generous by Swiss standards. The MCT for Zurich HB is 6 minutes…)
If the bus is on time you will be fine. And it could be even early. In Practice the way this works is that these busses just get filled with people coming form the train, and as soon as a bus is full it just leaves.
If you need to have your ticket exchanged do not go to the ticket office, as there are often long queues there. Go through the gates to the concourse at the head of the platform. There is a separate service desk there, called “Frecciadesk”, that is there specifically for rebooking passengers that have missed connections.