Not entirely true.. Ash fought two legendary pokemon, so as far as we take that the opening stated that Ash would fight two legendary pokemon, it wouldn't have lied. Just which specifically. I remember being fairly upset about it at the time as Entei is a favourite of mine. However, what this opening did was two things:
- 'Confirmed' Charizard's appearance when defending Ash from Reshiram.
If Charizard was shown in a different shot then that one, before defending Ash vs. Reshiram, then it would've been different. But this opening set it up to defend Ash vs. Reshiram. What we got was Pikachu defending Ash against Reshiram. So the context is vastly different.
That's true. Though I'd much rather see a combined effort of Pokemon stopping Reshiram than just only Charizad/Pikachu defending Ash from Reshiram to be fairly honest.