Is that really my decision to make?
Usually people can make decisions regarding their own health yes.
Unfortunately, with COVID-19, a person can spread it to others silently without any detection. It's not only about the health of the athlete, but the health of everyone he or she comes in touch with. Or even shares the space with. Olympics are especially great place for spreading COVID-19, with all the sweat, using shared equipment etc.
This is no longer a situation where an individual can take the decision by themselves (about COVID-19 risk),
as the individual has no way of getting the relevant information to do that decision. Because of asymptomatic cases and spreading it while not showing any visible signs yet. You can't make a decision based on information you can't get, and COVID-19 pandemic is one such case.
If this was a random athlete having a sprained ankle and they decided to still participate in olympic running events, that's his or hers decision. A sprained ankle does not spread. COVID-19 does, and unknowingly spreading it can get people killed.
If there was a 5-second COVID-19 test everyone could take whenever they want, it'd be a whole different story. Then olympics would probably be a go even for Canadian teams.