The one that says that Android is Linux therefore every Android device is a Linux phone (or tablet, etc).
This is often dismissed as a technicality but as every thread on so-called "mobile Linux" demonstrates, so-called "Linux phones" are judged basically on how well they can run Android crapware... just as "desktop Linux" is more or less judged solely on how well it can run Windows apps. Unlike Windows, however, Android is open source(-ish) and already a Linux operating system.
Most people who want to "switch to Linux" don't actually care about Linux, they just want Windows that doesn't suck. I imagine most people who want "mobile Linux" similarly want a non-sucky Android... which actually exists, unlike Windows.
If what you want is "Mobile Linux that can run Android apps" go install GrapheneOS or LineageOS or whatever.
Plenty of people who (I assume) are smarter than Trump don't understand that FOSS refers to freedom, not price. It's not a very good term and I don't like how widely used it is now.