Isn't saying Lufia I spoils Lufia II like saying that the original Star Wars trilogy spoils the prequels? "What?! Anakin becomes a bad guy?! Thanks for spoiling that for me, movie that is 20 years older!"
I'm assuming that the developer planned for people to play the Lufia games in the order they were released. I haven't played the game before but is it obvious playing Lufia II that it is a prequel to Lufia I.? Or is the fact that the ending is the Lufia I ending itself a twist, and that it's an intentional reveal? "Oh this is the beginning of the Lufia I! The stories connect!" If that's the case then congratulations to everyone for spoiling the game! It may only in fact be a spoiler if you know ahead of time that Lufia II's ending is Lufia I's intro.
You'd only know that Lufia II is a prequel to Lufia I if you've played the first game, because Lufia II plays its story extremely straight-forward (it's not like it starts with a flash-forward to the events of Lufia I and then flashes back to Lufia II). While the Lufia II ending is indeed a twist, I don't think the twist is meant to be that you're playing the beginning of Lufia I. If you've played Lufia I, you already know that because the character names and sprites are the same.
I think it just comes down to Lufia II being an infinitely better game than its predecessor, and it's the Lufia game that most people who have played the franchise have experienced. Lufia II has one of the best endings of any RPG from the SNES era, and I enjoyed it having never played the first game. I just think knowing the ending ahead of time really takes away from the emotional impact you should otherwise be having.
Look, if you want to play Lufia I, play Lufia I. But if you want to play both games in a way where both stories have their best emotional impact, I just suggest you play them in chronological order: Lufia II, and then Lufia I.