Again it depends on the gym and the instructor, a lot of Kyokushin techniques and training methods can be found in some Dutch Kickboxing gyms, mixed together with the western boxing and thai boxing techniques.
A lot of the punch-kick combos from Dutch fighters look more like Kyokushin than Muay Thai, especially when it's combos involving several punches and low kicks to the inside and outside of the leg. For example straight left + right hook + inside leg kick + outside leg kick is a typical Kyokushin combo. Nieky Holzken indeed has a background in Dutch Muay Thai, but he also uses a lot of western boxing combos and Kyokushin type of combos. Also look at how he throws some of his jumping knees from the spot without using his hands to grab, that's also something you see a lot in Kyokushin (maybe also in Muay Thai though, but I'd tend to think they usually clinch and push their knee through instead of jumping in the air from the spot). Same with his spinning kicks to the body and head, it looks a lot more like Kyokushin than Muay Thai.
Look at the fight at 3.38 for example, lots of hand + inside and outside low kick combos, it looks a lot like Kyokushin combos, also quite a few spinning kicks to the body. And then look at the hook kick to the back of the leg at 4.10. That's a trademark Kyokushin technique
You can also see a lot of Muay Thai techniques from him: like the way he throws his teep, how he quickly grabs the head + knee (obviously as proper clinching isn't really allowed in KB), also some kicks he throws from his front leg by switching legs first.
But to me this is why fighters like Holzen are so interesting to watch, it's a great blend of the best of all worlds: Kyokushin + Western Boxing + Muay Thai.