Ok first things first, I haven't played any Metroid games ever (shame on me and yes I plan on rectifying that) so sorry if this doesn't make sense or whatever. But I think the original Japanese can be interpreted in another way.
So the original Japanese is:
ゲームクリア後、ニューゲームスタート時にフュージョンスーツでプレイが可能な新な高難易度モード「FUSION」が使えるようになります。
Could be Nintendo is just wording this as vague as possible to get people to buy the amiibo. You can translate this as "After beating the game, a new high difficulty mode called "FUSION", where you can use the Fusion Suit, will be available".
MAYBE, this implies that after clearing the game you will unlock a higher difficulty mode regardless (that makes sense), and by using the amiibo, you can get a "Fusion"-version of this higher difficulty mode. So what that COULD mean is that the Fusion Suit is only accessible in the higher difficulty mode, which you have to unlock first by beating the game.
Once again, haven't played the original so if the Fusion Suit is something you should obtain during the game, then this interpretation doesn't make sense. But I checked the Metroid wiki and it says the Fusion Suit doesn't make an appearance in Metroid II, so this comes across as a fun cameo / costume with altered stats thing to me.
Edit: Hm so after reading more of the wiki the Fusion Suit doesn't sound like a cosmetic change but also something that would alter your defense, making the game harder. That would make sense to use as a new difficulty mode... I could see Nintendo locking that behind an amiibo... My interpretation might not really make sense then, so carry on