But it's not a reboot. It's just a passing phase like every other major Marvel event. Things "change the nature of the universe" in Marvel all the time. How is this any different? Sure, it could be a way to do away with the Ultimate universe because of poor sales and because most of the series there have run their course. But there's nothing being "rebooted" essentially. They're not going to restart books with a blank slate or redo origin stories again. They're just shaking stuff up to keep fans "excited" because that's how they've been doing the business for half a century now.
Was the original Secret Wars a reboot? Was House of M a reboot? Was One More Day a reboot? If you consider any of those a reboot, then sure, you might consider this a reboot too. But that's not how most people see it because the events that happened before still happened, and sometimes they are still referenced or can be brought back as story points depending on the writers. That's how Marvel rolls.
Every decade or so they'll start up a series of events, spanning many years, and then at some point they wipe the overall slate clean undoing a lot of the developments so writers don't have to be pigeon holed in certain story directions. For individual series, usually when a writer of particularly unique runs end their commitment, they have a chance to tie up their loose ends and retcon messy developments or kill off characters they created to return things to a status quo before another writer takes over.
The reason why people are saying this is not a reboot is because by the measure of what a full reboot usually means for entertainment, Marvel will never do it. There's no benefit to it. Whatever happens after Battleworld will be the result of the events of this Secret Wars, but everything that happened before it will still have happened.