Because the met level is not really the met level for generation 3 pokemon after being transfer to generation 4 to 7. When a pokemon is transfer from a gen 3 game to a gen 4 game the met level is override with the current level at the moment of being transfer, the same happens between gen 4 and 5. That means the met level of your gen 3 pokemon in a gen7 savegame is the level when they were transfer to generation 5.
Now the legality conclusion happens with the following logic. Your pokemon transfer level is lower than the minimun level to evolve, that means it was impossible to evolve it before transfer it to generation 5, and that means the evolution happens in generation 5, 6 or 7, the exclusive evolve move from generation 3 or 4 is illegal because the pokemon was not evolved in those generations.