If players can learn brand new positions 10 years into their careers, then a player can improve at a position from being awful to average.
Suggesting that it’s a stretch is ridiculous, in my opinion.
I specifically said earlier that guys that learn new positions late can definitely be expected to improve. At the same time, if you take a guy with no range at third, he's probably not going to be good in left.
My very first words on the matter were "he could improve defensively." My final words in the same post were, "Still though, he could improve. No doubt. It's unlikely, but possible."
I'm going to restate this once more and then walk away. Historical data shows us that guys tend not to show substantial improvement in their defense once they make the Majors. There are fluctuations in players' performances from year to year (although with improved data, this, too, is leveling out) but in general, when you make the Majors, that's about what can be expected of you moving forward defensively until you inevitably regress. Do guys buck this trend? Absolutely! Can Adujar buck this trend? Absolutely! Is it likely? No!
Especially because he has terrible range, and that almost never improves. I mean unless you're a fat guy and lose 50 pounds, how are you going to improve it? Range is a combination of quick-twitch muscle reaction, reading the ball off the bat, and speed (more so in the OF, but also SS and 2B). Very, very, very few players are able to make strides in these three areas to the degree that it makes a noticeable change in their range.
He can work on his footwork and his arm, and cut down on the errors and otherwise inaccurate throws. That would be helpful, and would be an improvement. If he does this, but his range does not improve, his defensive metrics will still be really, really bad because range is the thing that drags his numbers down the most. He could maybe get to below average? Last year was one of the worst defensive seasons in decades. To think he can get to average? Maybe he can do it! Who knows! Based on historical precedent, it's not very likely at all.
That's all I have. Good luck to him.