Luke did not train between movies, because he would have been back on Dagobah, but he wasn’t.
He doesn’t train with Yoda, it doesn’t mean he didn’t train at all. Pretty sure Yoda’s training wasn’t about choking people or using mind tricks 😉
(has he found that in the jedi books we later see in TLJ ? Or a Sith holocron he got somewhere ? Who knows…)
^ This
In TESB Luke:
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has to focus incredibly hard to pull his lightsaber to himself in the Wampa cave.
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is still pretty amateur with the lightsaber, being disarmed only 30 or so seconds after beginning his fight with Vader.
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is clearly rash and inexperienced, surviving Vader’s trap on instinct and raw ability.
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can’t hold his own as he gets toyed with constantly by Vader.
In ROTJ Luke:
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has had time to construct a new lightsaber (not likely an easy or quick task, especially considering there’s no one around to teach him).
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has learned far greater control of the force now and new abilities, one which he would have had to figure out on his own as Mala pointed out. He uses these effortlessly, including when he whips his lightsaber up into his hand from the Emperors chair.
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has far greater control over his lightsaber, so much so that combined with his new focus with the force that he can consistently while under pressure deflect/reflect laser fire now. He holds his own against Vader and ultimately beats him.
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comes across from the get go as a changed person, one that is now calm, focused and confident in his control of each situation. He has matured and wisened in many ways, people clearly look to him for leadership now where before that was Han and Luke was the whiney young guy. It is now Luke the Jedi Knight calling the shots.
To me it is pretty clear a not insignificant time has passed between TESB and ROTJ just based on Luke alone. I can’t remember but is it even known to any of the rebel characters where Han has been taken? I can’t recall a time in TESB where any of them would learn about this, so on top of the rebel forces needing to regroup and get themselves in order again after the evacuation from Hoth, the gang also had to try and find out which system, planet and group had Han after probably initially presuming the Empire to have him.
So I don’t have any problem with feeling that anywhere from 6 months up to a year has passed since TESB which is in line with the seemingly generally accepted year between TESB and ROTJ.