Kickboxing is an umbrella term.
Obviously Muay Thai is a very distinct art form with Wai Kru, the monkgon, distinctive scoring etc but it's my impression thatJapanese Kickboxing (not K-1 style kickboxing) is quite similar to what many would call international muay thai. In some cases they're arguably closer to each other than international muay thai and stadium muay thai.
A lot of the fighters in "K-1 style kickboxing" have a muay thai background now either way. I'm just guessing now but i think most kickboxers from eastern europe started out as muay thai fighters? Probably the same stands true for scandinavia and france.
I know there are very few fighters who truly are kickboxing specialist since there is many different
rules packages depending on what style of it is being promoted (dutch,american,etc).