Ok, I have an image of an A350 during flight preparation (engines off). No idea what the FCOM is telling...but even with ADS-B TRAFFIC Button ON--> no MEMO.
I think the description of the memo in the FCOM is wrong.
ADS-B is in lower picture still off, because ADS-B RPTG is off. And that is off, because the XPDR is in STBY.
The logic should be:
telling ADS-B TRAFFIC ON in phase 1,2,12,13 if still ON (both buttons).
telling ADS-B TRAFFIC OFF in phase 3-11 if still OFF (one of the two buttons).