How do we know your first statement is true though, did they test this sufficiently? Also, are we talking Canada? Ontario?...
It's the whole planet, and the level of testing has been massive when you think about it. Of all the things they need to research I don't think reinfection would be particularly hard to test and document. There are about six million known positive cases in the world at this point, three million of those have recovered so far, and all of those people knew they already had it. That's definitely a large enough group for the scientific community to know by now if people are getting reinfected.
I haven't seen anything at all where scientists are saying there are a
measurable number of people who have been reinfected, even at an incredibly low percentage rate. There are some anecdotal individual cases where some people have appeared to be reinfected, but most scientists believe it's because the virus was never fully cleared from their body, but low enough at some point to produce a false negative test, before a later positive test, creating the appearance of reinfection.
No question the science is still limited, and it's always evolving, especially for something as new as this. One of the interesting things about confirming immunity is it takes time to measure. So if immunity lasts two years we won't know that for sure until two years go by. But there are certain scientific clues about likely COVID immunity from other related coronaviruses, etc.
At this point there are lots of things they don't know about immunity in addition to how long it lasts. For example do people who are infected asymptomatically have the same level of immunity as someone who was really ill?
It's understandable people want guaranteed answers and it's true that science cannot provide that level of certainty, especially with something so new. For example, no one under 19 in Canada has died so far, but that doesn't guarantee someone under 19 won't die tomorrow. But what that
does mean is that at least so far, if you are younger your risk of dying is really, really low.
Likewise because millions of people have been infected with this virus and so far there is no scientific consensus that people are definitely being reinfected in measurable numbers, it's likely that once you've had it you will have immunity that will last for a certain length of time. An effective vaccine is completely dependent on that being the case.
So if you saw a sex worker who had fully recovered from the virus, is that a 100% guarantee she couldn't infect you? No. If someone who was 18 became infected, could they be 100% positive they wouldn't die? Likewise no. In both cases it's just fantastically unlikely given the incredibly large number of statistics we have in the world so far...