Unfortunately, yes. It originally wasn't confirmed to be a legendary until (I believe) 2017. Same goes for Type: Null.
(I only say "unfortunately" because I don't like the sudden change, nor the fact that it's legendary in general.)
And a correction for Micah: it's NOT a pseudo legendary. They have completely different definitions.
According to Bulbapedia, "Pseudo-legendary Pokémon (Japanese: 600族 600 club) is a fan term commonly used to refer to any Pokémon that has a three-stage evolution line, 1,250,000 experience at level 100, and a base stat total of exactly 600 (before Mega Evolving). Pseudo-legendary Pokémon are often more powerful than many other non-Legendary Pokémon due to their high base stats."
If Type: Null and Silvally were pseudo, they would have a third Pokemon in their evolution line, it wouldn't have the official "legendary" status, and they'd have a dual type. I think the only reason why Silvally got the special background is because it wasn't confirmed at the time, so it might've been a temporary mythical.
Silvally is a weird legendary exception, but it's not pseudo.
(Sorry for the long correction- I'm kinda just the correcting kind of person.)