It seems they more-or-less automate PR card renewals - but only for some subset of applications. Presumably those are (for some subset* of) the straightforward and easy ones.
Note that 'mailed' may be optimistic. Probably this is equivalent to sending instructions to some other office within IRCC to print, send to mail room, put in envelope, package up with other ones, and then hand to Canada Post. It almost certainly means it will not take the 2-5 days more or less standard for regular post - probably more like 1-3 weeks.
But that's still pretty fast.
*I'm referring to different subsets because they (or their computer algorithm) decide what's straightforward and easy - you might think your application is 'simple' but hit some criteria or more that kicks it out to manual clearance. If that happens, doesn't necessarily mean it's a problem file, just won't be done automatically.