True, but Yoda didn't fess up about dear old dad until Luke asked him point blank. Mentioning someone who was likely too far away to be of any help in the current situation wasn't what Luke needed to hear.
If Yoda couldn't sense any survivors after Order 66, going back to rig the Jedi Temple transmitter to send a warning to stay away seems pointless.
Ahsoka left the order, and they cancelled her membership, so it can still be true from a certain point of view. ;)