None of those say anything about handguns and how many were handguns. Many rifles (arbitrarily) were restricted, too.

Edit: also police have their handguns lost or stolen a lot more often than you think.

Either way it does not justify this law.

Edit2: I looked for your claim that domestic sources disappeared and haven't found anything supporting your claim.

Also if you want quote those people here is another one like them from a few years ago.

https://crimecanada.ca/rcmp-report-most-crime-guns-domestically-sourced-in-canada/

This article is flawed as fuck, but apparently handguns still make up 9+% of domestic crime handguns despite sales having been halted fully since November 2022. Before the most I could find was roughly 15%. Which was in the same ballpark it has been for decades. Straw purchases for Canadian handguns have been impossible for a long time and with people unable to buy new ones unless those straw purchases were the only ones buying your assumption is impossible.

And again, that specific article I mentioned is bullshit. It purposely excluded Ontario and Québec.