American conservatives are famous for never giving the slightest fuck about how other countries do things, hence it seems to be somewhat opportunistic to do that here when it looks convenient.
Also, I find the text misleading.
Mail-in voting is common or even the most common form of voting in several European countries. In Switzerland, 90% of votes are submitted by mail. Germany recently had an election where all votes were by mail (due to the pandemic); outside of the pandemic, 25+% of all votes are typically cast by mail.
I don't know about all European countries, but besides Germany I know that UK, Spain, Austria, Switzerland, and Denmark have mail-in voting for everyone (not just selected groups), hence it's not at all unusual.