If someone buys cryptocurrency/ owns cryptocurrency before moving to canada then that means it is his capital before he become resident of canada so he should not be liable to pay tax on it in canada as this is the income from another country. no?
This is a double taxation issue.
I have not seen a double tax treaty between Canada and another country.
There are a lot of nuances here. For example, in the country where you live, the tax is 10%. In the country where you are moving, the tax is 15%. The tax office may require 5% tax from you.
Another point is the time when you cease to be a tax resident of the first country and become a tax resident of the second country.