Its not officially canon to the lore of Overwatch. Yes there was a reported exchange with a Blizzard Customer Support Team member and a player but that person is not a member of the Overwatch Development team and to be more specific, not part of the creative writing staff for Overwatch. Officially in the story progression it is very likely Mercy and Roadhog have yet to see each other directly (that may change in Overwatch 2).
There are stronger lore connections that connect Genji and Mercy together including what is seen in the Overwatch Comic Reflections (see page 11) and certain voice lines that have appeared throughout the game.
Now to more serious business, if you want full posting access to the forums, you need to make sure your BattleNet account is activated with a license of Overwatch or linked to a game console that has played Overwatch. Please follow this troubleshooting guide for assistance.