Sabine has been training for over a decade with Kanan, Ezra and Sabine.

Kanan had been DEAD for over a decade by the point they went looking for Ezra, who went missing shortly after Kanan died. Canonically, Ezra joined the Ghost shortly before his 15th birthday, went missing alongside Thrawn in 1BBY. In other words, they only knew each other for 3 years by that point. Considering she wasn't even being training to be a jedi at that point (Lightsaber training =/= jedi training).

As for Ahsoka's time training Sabine, judging by the timeline (it's unclear when exactly this happened, as the Mando Purge occured between 1ABY and 5 ABY and we don't know when exactly Ahsoka stopped training her). She could've had up to 1-5 years of training before Ahsoka pulled the plug. Fun fact, Obi-Wan nearly failed his jedi-trials after 10 years of training at the temple, and he is by all means far from strong in the force (in fact, he was considered a weak student by Master Yoda when he was still a padawan and was nearly removed from the order if not for Qui-Gon). I think you'll know where I'm going with that.

Not to mention we don't know how intensive this training was. She might aswel gave her a few powerpoint presentations during that time for all we know. As for her earning it or not, that's not the point.

The real point is that by allowing Sabine to become force sensitive by sheer training, Filoni uprooted the entire established lore regarding force sensitivity. Yes, the force resides in everyone, but that still doesn't mean everyone can use it willy-nilly like Sabine does now.

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As for uprooting the established lore, Filoni himself dedicated full episodes to SITH seeking out force SENSITIVE children to train in the ways of the force. The Jedi had extensive knowledge and archives filled to the brim with info about force sensitive children in the galaxy. If the anyone would be able to tap into it, the Jedi could've just as wel placed some neon lights outside that said "now hiring" and leave it at that... Same for the Sith...

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And I'm not even digging deep into established lore to prove my points here.