As Colours mentioned, I believe this isn't remake but its also not a sequel, at least with current knowledge. IT's got a similar vibe to BW2 of course, with the two games and the fusion of what is most certainly Necrozma with the SM mascots. The problem with it being sequels is that, as of now, there has been no indication that these games occur after the events of SM. In addition, in the previous sequels, BW2, there was an antagonist that was present in the first games, Ghetsis. In SM, all the main antagonists were dealt with in some way, Lusamine is in KAnto, Gladion is reforming Aether and Guzma, besides the fact he isn't capable of hatching such a plan in the mold of Lusamine, Ghetsis, Cyrus, etc..., is on the road to reform, kind've. So where could the new antagonist come from? Plumeria is a possibility though unlikely since she was pretty loyal to guzma and mainly just wanted to protect her "brothers and sisters". Faba seems a bit more likely but with Gladion keeping a close eye and Faba not being as sharp as you'd expect, I find this route unlikely as well. So unless the plan is to change Plumeria and Faba's personality as well as expand their role/introduce a completely new character, the direct sequel route seems very unlikely to me.
As for it being a remake, besides it being way too early compared to previous remakes, the phrase "alternate storyline" holds a lot of meaning. Now Platinum, Emerald, and Crystal are all technically alternate storylines, features stayed the same pretty much besides the sootopolis gym leader/champion situation. In addition, it was made a point to say it was an alternate storyline, which leads me to believe it will be very similar to SM but with changes such as, (this is an example and not what I think exactly) gym leaders instead of trials (bad example but makes my point). The best I can explain this is through a comparison to The Flash and Legends of Tomorrow TV shows. I like to think US/UM are going to be on a different timeline, like ORAS was in comparison to RSE.
Simply put, I see these games as existing on a timeline separate from SM (maybe the same one as ORAS?) with enough changes to seem like a sequel a la BW2 but enough similarities that call back to its originals a la ORAS.
As such, I think this will essentially be a a third version but with two version of said third version. In addition, as some have noticed the characters look a bit different, so this is where my alternate timeline piece comes in.
Hope that all made sense, I'm still trying to wrap my head around some bits of this too :)
Edit: I think Rivvon, evil chameleon and others explained this as well but in a lot less words than myself, as I definitely overexplained xD