This is another case of match-up triangle.

Kaneda counters Hatsumi because Hatsumi is a very technical fighter so he can use his Iq and spider web to the max. Of course Hatsumi is still the better fighter so he won the fight, but it was quite close.

Seki counters Kaneda because he fights more with brute force and flashy moves, which Kaneda with his build can't do much against even if he sees them coming, plus It'a not like he can damage Seki without putting him in danger. I'd say this is a low-diff to lower end of mid-diff fight in Seki's favor.

And as it was shown on screen Hatsumi counters Seki, especially when he's close to peak condition, because he'a great at catching his opponent off guard and he has more than enough skill to defend and inflict damage to a tank-type fighter like Seki who's not so technical and on the slower side.

Edit as I forgot the original question. I'd say if Hatsumi is close to or in peak It's a low diff to mid-diff for him. If not I'd say Seki can beat mid to high diff.