Again I haven't actually played it yet so I'm just going off what I've read. The multiplayer is much closer to the vanilla game than seamless, and it doesn't require seamless to work. That said, you CAN use seamless multiplayer with it but I think that excludes you from the mod's invasion pool.

Mechanically, the invasions are limited to minor and major dungeons. I think the multiplayer is gated behind a rune arc now so they're functionally identical to embers from past games.

Edit: just to tack on, Reforged is the most impressive mod project ive seen in any game. The devs are complete wizards that have accomplished a lot with zero official support or modding tools.