Closing the loop on this: I understand from some other posts that you cannot apply online for the PRTD without choosing the outside Canada option, and of course you must swear everything there is true. (I'm still unclear on whether you can choose to apply online or not, but some offices apparently not accepting paper apps)or even makes it impossible to apply for a PRTD without choosing the 'outside Canada' option (and also swearing to that effect).
So: IRCC may not physically block you from applying for PRTD while in Canada - so it's technically 'possible', that doesn't mean it's allowed or a good idea, because you may have to explicitly make a misrepresentation to do so.
So again: you can't apply from inside Canada without making a misrepresentation. Technically possible, yes, but that doesn't mean it's a good idea.