UPDATE - Hmmm. From comments:

https://forums.opera.com/post/54173

and

https://forums.opera.com/post/54279

it would appear that something is indeed going on in the realm of Opera auto-updating Presto Opera versions to (currently) Opera 25. In both above cases, the updates were unsolicited by the user himself; and in the second case, the user states he declined the spontaneously-offered update, but it appeared to have completed anyhow after he restarted his computer. Coupled with the tenor of the announcement about upgrades being offered to "some" Presto users, it appears to me to be likely Opera is selectively using Presto Opera's update query to provide auto-updates to some users, but not necessarily to anyone who simply manually checks his update status from within Presto. (Such selectivity is sometimes used by software providers to keep from overloading the update servers.) That kind of updating, in turn, may well have started at least sporadically/experimentally prior to the announcement made today (15 October). Moreover, it seems that this updating process is not behaving either consistently or appropriately in at least some users' experiences. Both the comments refer to users running Opera in sandboxes.

I'm still awaiting Opera's "official" comments on all this, but that updating process's behavior has clearly been either inadequately tested in the real world or the updating process's design is inappropriately ignoring users' choices and/or Presto browser update settings.