i agree it's safe

think of brass like a rubber gasket, that seals the gap between the breechface and the barrel. the real danger comes from damage nearer to the base of the cartridge - if i saw that dent right near the bottom, i probably wouldn't fire it

it's not ideal, but not terribly uncommon, for case necks to split. the only danger is that it might leave a brass fragment in the chamber which will inhibit seating of the next round, but no gas will escape because the breech is still fully sealed

also, just to be clear, 'if it seats it yeets' is not actually good advice, it's like saying 'the first rule of gun safety is to have fun'. it's the sort of fun thing that people say here to each other without realizing that beginners might take it seriously. .300blk will seat very nicely in a 5.56 chamber, but nobody wants to pull the trigger on that