I wrote that you can't keep an Ontario photo card and a driver's license and they'll cancel it anyway.
I guess that was ambiguous - or at least insufficiently clear - that I meant you can't have both an Ontario PC and an Ontario DL. They will (and of course can) cancel one or the other.
Yes, of course they cannot cancel a foreign driver's license (at least that I'm aware of - note that I think they possibly could advise some US states that the driver now has an Ontario DL, because they share info about violations, fines and points with some states).
Yes, it seems like a weird requirement. Note that I think that the main issue between Canadian provinces and US states are concerns that a driver might try to hide their 'main' DL (or DL identity) to evade certain driving and insurance penalties and the like.
So I would explain it this way: consider that actually using a 'foreign' DL or DL from any other jurisdiction in Canada or USA may and quite possibly will be considered a form of identity or insurance fraud.
This is especially true if eg you used a foreign driver's license in Ontario for any driving-related purpose. And since you signed a document when you got your Ontario DL that you are/were surrendering or did not have any other driver's licenses, you are potentially liable if you do so.
Consider yourself warned: I think they treat this very seriously.
As for whether eg you use a UK or Indian or French or Brazilian or Russian or whatever DL when you are in that country, or travelling elsewhere (outside Canada/USA): it is probably a hypothetical risk and with very few practical implications.
However: one hypothetical you should be aware of - if you got in an accident in 'home country' after receiving Ontario license and then had an accident covered by insurance that was not reported to insurance comapny in Ontario (when/if required), the insurance company might consider that a breach of your insurance policy requirements - leaving one liable for large monetary risks. (In provinces where govt does car insurance, that might also be an offence).
Not a lawyer - just explaining what I think I know about this and the policy reasons for a policy that might/does seem odd to some. Do your own research or hire a lawyer if truly concerned.