Is it a bad idea to put a 60 A breaker in a 100 A panel?
The instructions are CSA-approved (or UL-approved; UL is now licensed to approve in Canada thanks to trade deals like NAFTA and GATT, thanks Bill Clinton).  They are requirements and must be followed, unless you obtain an AHJ waiver.
The reason the UL-approved instructions require at least a 125A service is because a 100A service would not leave enough headroom remaining to reasonably power a modern electric house with presumably at least one other large electric appliance along with normal lighting and small appliance loads.
Subpanels are irrelevant to this type of service calculation.  The old joke &quot;I can't be overdrawn, I still have checks left in my checkbook!&quot; really nails the difference between breaker spaces (and how they are distributed in subpanels) -- versus -- ampacity drawn by those loads.
Your house's calculations are based on the ampacity of the various loads (as well as square footage and some other odd stuff that goes into a proper load calculation).  The arrangement of panels and spaces doesn't enter into it.
That said, you have to do another load calculation per subpanel to make sure you are not overloading that subpanel's feeder.  But the 125A admonishment in the instructions does not apply to that.
TLDR: you're fine if your house's service is &gt;=125A and it won't bust load calculations for your house or sub.