Yeah....given that the info says "This is the beginning of the adventure towards the apparently infinitely vast Pokémon world", it very much seems like a reboot to me now TBH. Why would the info state something like "beginning of the adventure" if this was just a continuation of Ash's long journey from the previous sagas?
Guess at the very least Ash (the one whom we know throughout the course of his journey through 7 regions & 22 years) is being finally being written off. At the very least, I can be pleased knowing that he's leaving on a high after finally winning Pokemon League after so many years, after finally achieving that seemingly unachievable target/accomplishment which has been portrayed as a primary step in his goal throughout the course of the anime.
But what I'm really glad about here is that: the repeatitive, rigid and tiring formula in the Pokemon anime is finally getting a change, as it seems like. Ash losing the League, Oak'ing his regional Pokemon at the end of the saga & going off to the next region was a formula that was getting really repeatitive & predictable throughout all these years. IMO, the Pokemon anime really needed a change in formula at this point.
So if this series indeed a reboot with Ash's character & story being rebooted alongside, I'm not gonna be upset about it even one bit.