True, but Fido does have its own MNC (370), distinct from Rogers' MNC (720).
MNC, or Mobile Network Code, is a 2- or 3-digit code permanently imprinted in a SIM card which, along with a 3-digit MCC (Mobile Country Code), is used to identify the subscriber's "home network". Every country has its own unique MCC. Every network has its own MNC which is unique for the country in which it operates.
SIM locks essentially work by reading the MNC and MCC from the SIM card and comparing them to corresponding codes embedded within the phone. If they match, then the phone is allowed to use the SIM card. If they do not match, then the phone will reject the SIM card.
If the iPhone is smart enough to keep a list of multiple permitted MNCs (one for Rogers and one for Fido) then it might be as simple as swapping a Rogers SIM card for a Fido SIM card. Otherwise, you'd have to unlock the phone before you could swap SIM cards.