Is it safe to hang a lightweight ceiling fan from electrical box ears?

Of course, Home Depot instructed me to use a standard junction box when I went to get parts for the guys installing a ceiling fixture. It’s securely mounted across two studs.

I wouldn’t think twice about installing the ceiling fan into the existing box, but on the instructions for the Saf-T-Brace (a ceiling fan-specific junction box and brace) I bought for a different ceiling fan, it specifically says “Install fan or fixture…into the threaded inserts, NOT THE ELECTRICAL BOX EARS”

The fan I have is < 20 lbs. Do I need to replace the whole electrical box with a special “ceiling fan” box?