It's not a myth. The heat pump at my house works great, but it shuts down around 12F (-11C). We don't often hit that temperature in a MA winter these days, but it does happen. Oil burning for backup.
* https://www.mitsubishicomfort.com/articles/keep-warm-this-wi...
* https://mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.c...