The measures were implemented on February 15. They do not apply retroactively.
Going to your example and understanding the information that was shared by the RCMP with the financial institutions, I must admit that based on the knowledge I have, I think it would be unlikely—I can't imagine—that someone who gave $20 three weeks ago, or even $20 post February 15, would have been captured by a freeze. It's not impossible in view of the order, but in view of the exchange of information and the focused approach that was taken to stop the illegal funding of these activities, it would appear to be unlikely that this occurred, but not impossible.