The thing with the Eevee anime-theory it just seems like people are trying to form a pattern when there's simply just a line. Eevee already has a tonne of evolutions as is, and none of them have been given the consistent development that Chloe's has had (other than Serene's). I think people are just trying to find a pattern when there simply isn't one. This might not make sense since I'm not good at explaining things without writing literal novels so here goes.
Chloe's Eevee has been given this huge arc spanning over a year about it's evolution (she caught it in December, 2020), I don't know, call me cynical, but I just don't think the anime would outright tease something like that for the upcoming generation over such a long period of time... Gary and Serena both got their Eevee's when we already knew Umbreon and Sylveon existed (Gary "caught" his during the last episode before the Johto arc started, and Serena caught hers during the Kalos arc), the only exception may be May (I'm not too sure when Glaceon was revealed), but if they really wanted to showcase Glaceon that early there's no reason they couldn't have evolved it earlier. Instead, it evolved off screen and debuted as a Glaceon more than 2 and a half years later and appeared in about 3 or 4 episodes. They could have spent that time teasing Eevee's future evolution into Glaceon, but they didn't. This would have been easy enough to do since the AG anime already showed off a tonne of Sinnoh Pokemon, including Brock's Bonsly, James' Mime Jr, which they all caught during the Hoenn anime, and lets not forget May's Munchlax, which she caught a year and a half before Diamond and Pearl even released.
The only time I can remember this happening with the Anime is with Ash's Rockruff. He caught it at the start of the SM anime and it evolved to show off the new Lycanroc form for USUM (I believe the episode aired the same day/day after they first revealed the Dusk Form, and the anime episode a week before teased it). Rockruff first appeared in November 2016, but Ash didn't catch it until February 2017, and it evolved in August 2017... Depending on how you time that, it's only a period of 6 or 9 months to tease a new evolution, not 1.5-2 years that Chloe's Eevee is getting! Not to mention, Rockruff never had an arc dedicated to it's evolution either.
If the theory was to hold true, I don't get why May wouldn't have received a Glaceon during the Hoenn anime since they'd already established a precedent with giving a lot of the main cast Sinnoh Pokemon. I don't know, I've always thought Chloe's Eevee was more a reference to Let's Go. I mean it even has the female tail gender difference which first appeared in those games. Plus, in SwSh, Eevee that can G-Max can't evolve, and we haven't seen a G-Max Eevee in the anime yet... Just seems like Chloe's Eevee is leading up to a reveal that it can G-Max, not that it's going to evolve into a brand new Pokemon
That's just my 2 cents worth. I don't want to stop people from speculating but I personally don't think it holds any merit.