I think what they're trying to get at is that there are no fittings attached to the tee to inhibit or promote ovalization in the tee, and that real world SIFs may vary if you attach, for example, flanges (to inhibit ovalization) or elbows (to promote ovalization).

It does not state "two diameters of piping with equal thickness to the tee."

Most forged tees are already thicker than the pipe, at least in certain parts, and we generally don't take credit for this fact, because exact thickness rarely is known in advance, unless you're on really good terms with a fittings manufacturer.

However, with that said, if you had a custom application with SCH 80 piping, and you said "make me a SCH 160 tee, instead," I'd absolutely input it as SCH 160.