Soft canon?! Never heard that one before.
I think nobody found Krall and Jaylah. There was no Starbase there and the nebula/asteroid belt hid them until a planet killer came along.
Well as per STO itself -
STO Website said:For Star Trek, canon is defined as anything that has appeared "on screen" - either in one of the six television series or in one of the twelve Star Trek movies. Star Trek Online, as a game, is considered soft-canon and any of its stories might be contradicted by future Star Trek television shows or movies.
As for never being discovered.. there not ever being a starbase there - I would say that's a variable. We don't have any information regarding the nebula, the planet, or frankly the ship until STB.. Finding their place in Prime without the events of the Kelvin timeline would suggest that as i said in an earlier post, the possibilities are endless.