Semi-retroactive achievements for nether bosses

Since some players have already defeated the nether bosses at certain nether realm levels the achievements are impossible to obtain without another save file reaching them.

Is it possible to provide the achievement if the player defeats the nether boss or visits the nether realm and clears it.

Example
Player already has defeated nether realm 5 boss.
Player reenters nether realm 5 and progresses to realm 6 through “statue/crystal”.
Player receives the achievement since they cleared realm 5.

They are actually possible to obtain even if you’ve defeated the bosses already - the achievement descriptions just don’t say that because they’re made with a brand new player’s perspective in mind. After nether realm level 150, the bosses loop back to the way they were starting at realm level 5 - just with higher levels/gene strengths, of course.

So defeating the boss at realm 155 would give the realm 5 boss achievement?

Correct!