You asked "what's the point?". The point is to deter travel and spread, using the measures possible; now you've just shifted from what's-the-point to "wait it's not perfect!". In full sarcasm mode, thanks, chief - brilliant insight. Deterrence that is imperfect is still deterrence; trade-offs exist - such as closing 'loopholes' where they exist is sometimes not possible, and sometimes - takes more time. (And then different gaps or loopholes or found - whack-a-mole). Every policy maker knows that few measures are 100% effective, and that getting to 100% may be undesirable, unfeasible, or too expensive.
In reality - let's wait and see what happens. Often what happens is that there are discussions more or less behind the scenes - and I wouldn't be surprised to see Qatar introduce additional measures and restrictions on entry/travel through Doha, possibly at the (partial) encouragement of Canada. And also possibly Canada - for example - limiting the number of flights from Qatar - or warning that Canada doesn't want to impose such a measure but could and hoping Doha gets the message and takes measures on its own.