I did the job properly and got another PCR test for this trip. My passport was checked twice before they administered the swab.
However, just for research purposes, I found a website which converted last year's certificate from pdf to Word format without messing up the format at all. So it would have been ridiculously easy to edit the dates and save as a new pdf.
So, now there is a new variant, the Thai test on arrival system actually makes some sense. Particularly with a high risk of fake certificates from some countries.