I would like to know for speedrunning games horizon zero dawn where there are cutscenes, can you skip those or do you have to watch through them, the rules don't say you can or can't so I would just like some clarification?

Ask in the game forums.

^ Generally good advice, but this is very much a common sense question. If the game lets you do it (and cutscenes in Horizon are always skippable), yes it's allowed. No game with skippable cutscenes has or will ever require watching them.