Ahoyoo was known to be a good player, particularly with bombs (his other level, bombs5, is the same kind of tricks).
In 2017, there was no emulator capable of running SMM1 properly, and there was no controller for it to TAS with. So in order to TAS the level, he would have had to build an Arduino, learn to code, detect somehow when the level starts, send inputs to the USB port, enter the right inputs, and try and work out timings which are actually very difficult even in old systems, and very time consuming.
The best players currently are making progress on the level with around 20 hours of practice. Ahoyoo likely had hundreds of hours on this level alone (as the creator even, just to make the level), and it’s said he spent thousands of hours playing the game obsessively. Spending more time to TAS would have been counter productive. If not impossible without an emulator (for reasons that are very technical TAS’ing a modern system is close to impossible without a camera and some AI).
Now there is a theory that he could have cheated the video (with editing) and cheated the submission process, but the cheats for submission were not widely known at the time (2017) and Ahoyoo wasn’t known as an editor. So he would have needed help etc etc. That leave traces on forums that people likely would have found by now.
Being a good player and obsessed at the game as he was, it’s more likely that he beat it fair and square, the way people are getting closer every day to beat it. So with all the evidence we know that accusation doesn’t hold.
Edit: well that didn’t age well. I guess he DID have early access to TAS tools.