Now that I think about it, the only directly figure skating-related thing that Victor taught Yuuri as a coach was a quad flip. That’s it.
Yuuri could do all triple jumps (including the axel) and a quad toeloop before Victor became his coach, and the person who taught him the quad Salchow was Yurio, not Victor (although I have a feeling the latter would have helped him polish it).
In terms of skating, Victor was mostly responsible for his choreography, apart from the quad flip, since Yuuri could already perform elaborate, beautiful step sequences and spins much earlier in his career (if his practice session with Yuuko where they mimicked Victor’s programs are any indication of Yuuri’s predisposition to skate beautifully, which I think they are).
But what Victor gave him as a coach was not figure skating skills and I think that’s the point. Yuuri already had the skills, he was already amazing. But like the unreliable narrator he is, he refused to acknowledge it, much preferring to call himself ‘a dime a dozen figure skater’ than actually realizing how good he really is. This can mostly likely be blamed on his perfectionism and his low self-esteem (both traits quite reliant on each other). What Victor did was helped him find the confidence to say “Yes, I want a gold medal at the GPF, and fuck yes, I think I’m good enough to get it” (not the exact phrasing, but you get it). I think it would even be fair to say that the reason Yuuri even wanted to learn the quad flip was because he finally considered himself worthy enough to succeed in it. It was Victor’s - his lifelong idol’s - signature move and Yuuri was finally able to say “I want to do that and I can do that.” Talk about quad flipping everyone off.
And Victor’s choreography also cannot be dismissed, as in the programs he created for Yuuri, he did put a heavy focus on complex step sequences and spins that really let Yuuri’s strengths shine.
And even aside from all of that, Victor was what Yuuri needed for a long time - mental support. Of course, he’s had his family and his friends and Minako and Celestino and Phichit but even with all that he still didn’t feel loved or appreciated. It seems like Victor was the first person who actually cared, who took the time to sit down, understand him and try to help him with any and all issues he had with himself. Victor went out of his way to learn about him and take him and his problems seriously, and this was most likely what made Yuuri realize that his friends and family and everyone else had been trying to do similar things - he just kept shutting them out, keeping to himself.
So while it may seem that Victor hasn’t given Yuuri much in terms of actual skating - only a quad flip (which can actually be a powerful weapon, even if it seems like only one jump) - he has been a catalyst that helped Yuuri realize things about himself, come to terms with his shortcomings, notice his strengths and finally skate closer to his full potential than ever before.
And while of course Victor can’t and shouldn’t be credited for everything Yuuri can do and has achieved, all of this certainly makes me think that Victor is indeed a good coach.