You are allowed to use your small ring for both TTs and Tris. They are making an overall point- in a TT you are going all out(same idea for Tris) and there is no point to be in your small ring unless there is a massive climb.
In all aspects of road racing(crits, road races, TTs, Tris- I have no experience with MTBs...) You will start off in your big ring and only drop to the small ring if needed.

In TT I start in the biggest gear I possibly can from the holding start and still get a good sprint off the start line(so start somewhere sort of easy in the big ring and then spin up to a bigger gear). 99% of the time in a TT you will not shift, shifting raises your heart rate and you lose wattage.