"You can indeed submit TSA approval on arrival in the US - but you will then have to sit and wait for as long as the CAA and FAA take to process the application and no training can be undertaken in the meantime till the TSA say so. So that would be a complete waste of time.
Plus- suppose the TSA were to reject the submission?
Far better to get it submitted and approved before getting on a flight to the US."
On the same thread I believe your quoting I also pointed out that TSA apporval can be transmitted same (Working) day and that training can commence as soon as the TSA have acknowledged receipt of the relevant information. And IIRC this was disputed despite it being clearly stated on the TSA website.