Cheronians live up to 50,000 Earth years (literally said in dialogue in TOS, they even specified it was "your years", absurd as it was).
The cadet in SFA only puts us at 4 survivors: Bele, Lokai, Virgil, and SFA cadet, who may have been alive since the 23rd century because with an over 50,000 year lifespan, 900 years is relatively nothing to her.
Does it continue to undermine the ending of "Last Battlefield"? Of course, but the fact is that 4 survivors out of a population of say, 8 billion or whatever (they never said Cheron's population so I'm just making a guess based on Earth's current population) is still not a lot.
It undermines the loss of Cheron but honestly Star Wars has it worse with more survivors out of a couple of thousand from the Jedi purge (Grogu, Ahsoka, Barriss, Quinlan, Cal Kestis, probably missing some more)