So from what I understand about the process of the TAS's you make is that you TAS a section and then splice the video to make it look like its a single input file? I dunno, but I feel like that would allow you to download a video from another person and use it in the TAS...
When you record a TAS, you do it frame by frame and with save states on an emulator. All your inputs will be saved in a seperate file and in the end the whole run will consist of this single file. This input has to be provided to show that the TAS is legit, and if you do, there is no way to cheat either.
Their any% run on youtube is however spliced together when it comes to the encode itself. They never provided the actual input file, which is quite suspicious to be honest... But hopefully they are going to with this one, otherwise it too can't be fully trusted.
It is possible to edit the input file, as you can open up the file and view the input. Though when you edit it, depending on the game, there is a high chance that the run desyncs because of changes in the RNG and similar.