Yep, they right. DVa's not a tank tank, if that makes sense. More like a heavy duty flanking DPS. Lately, I've been trying to stick with the healers or payload until you see someone in a vulnerable spot. Dive onto that Widow up on the catwalk all by herself. Get the Genji trying to flank. Don't boost into the middle of the enemy team, unless you're willing to die to get that Mercy/Moira or other high-value target.
When you're with your Mercy, follow her when she GA's for a rez, and be prepared to protect her, preferably with your defence matrix.