Of course, it's a fictional series. They CAN just hand Ash strong pokémon without him ever having to train them. Doesn't mean they SHOULD.
That's the thing though.
That's ALL they can do is hand Ash or any character strong Pokemon. Because a Pokemon's strength is ENTIRELY determined by the writer. Ash can capture a Pikipek and it can defeat the strongest Pokemon in the world even if its otherwise immune to flying type moves, and Pikipek can win with just peck. Likewise they can give Ash a Guzzlord and it loses to a caterpie sneezing on it.
The writers have absolute full control over how anything is written.
And on top of that the show isn't a 24/7 hour livestream of everyone's life at EVERY waking moment.
What I'm saying is how do we know Poipole didn't just battle Necrozma over and over, beaten with an inch of its life ultimately evolving into a Naganadel, in a MUCH more efficient training than Ash or any STRONG character can "reasonably" come up with.
We don't know, and I guarantee even if they did say that Naganadel trained with a powerful legendary Pokemon, people would still dismiss that as B.S. Why because people believe everything can be shown on screen of a 22 minute episode? Let's condense everyone's life of 24/7 into a split screen sped up to 22 minutes, so whatever that works out mathematically, and then maybe people will stop complaining when they see Poipole in the Ultra Space battling and training constantly.
I mean here we have a character that effectively DOES NOT exist who was in the space of non existence, somehow reached the point of evolution and learned thunderbolt.
The idea that a Pokemon can't be strong or trained or be better off until we see small progressive steps at every waking second of its entire lives, seems very illogical no matter how you look at it.
Either you try to explain it in-universe ignoring that its fictional (which was my point about people being naturally better than you) or we strictly look at it PURELY from a fictional point of view, where ultimately you have to accept things, because Pokemon isn't a documentary 24/7 series.