The fact he can draw in and perfectly balance the natural energy prove he completed the training. The duration of the mode variies depending on the amount of natural energy you're capable to draw in your body. Naruto for example was capable to draw in more than even with the oil [
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], which allowed him to maintain the mode for 5minutes at most(bar when using 2 Rasenshurikens that completely take away his sage chakra).
Minato's Sage Mode not lasting that long simply means he doesn't manage to draw a lot of the natural energy inside the body(not as much as Naruto for example that's for sure), but it still prove he completed the training. Even Jiraya who couldn't enter sage mode himself did [
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]
Just like he never tried to use senpou rasengan despite having already marked Obito? Just like he didn't try to use clones? Just like he didn't use rasen senkou chourinbu kousanshiki despite saying in the war it's his marked jutsu? So once again, a character not using a move at one point doesn't prove he can't