Is there any evidence, that is considered canon, which would prove that it was the whole planet which was destroyed?
The canon is pretty much "Full Circle". Oma saved the villagers by helping them ascend and Anubis fires his new super-weapon. Back on Earth the team repeatedly try to dial Abydos without success until they finally make a connection. Skaara explains Abydos was destroyed, and that the Stargate will disappear when they return to Earth.
Since they were able to go back to the planet, It wasn't "Death Starred" into a billion piece, but all life had been wiped out and any new clues to finding the lost city have been nuked as well
EDIT: In universe why would Oma lie?
EDIT: Out of universe answer
Full Circle also setup for Michael Shanks to return as one of the SG-1 core cast as de-ascended Daniel Jackson. From a story perspective they needed a reason why he'd risk being de-ascended.
The writers could have put Earth in danger again, but they'd already done that for several season finales. They also wouldn't be able to demo Anubis' cool new super-weapon as destroying Earth = no more show.
The answer - Abydos! It also conveniently lets them tie up some character stories such as Skaara's. He was a recurring cast member in the first few seasons, but got put on the back-burner as Goa'ulds were defeated, and new villains introduced.
The other choice would have been just to not be able to dial Abydos and have a character come in and say it's been blown up, then have Oma show up and explain what happened, but this breaks the "show, don't tell" rule.
By going back to a desolate planet, the TV lets the audience see the "total devastation" of Anubis' new super-weapon, and shows that Skaara and the others survived. It also clearly wraps several story-lines they likely were never going to explore further.
Out of universe it's all special effects anyway. There's no in-episode hint that what the teams sees at the end isn't "real".