> Where in The Free Software Definition https://www.gnu.org/philosophy/free-sw.en.html does it require the user to be able to run the modified versions on the same device?

Quoting from the above link:

Freedom 1 includes the freedom to use your changed version in place of the original. If the program is delivered in a product designed to run someone else's modified versions but refuse to run yours — a practice known as “tivoization” or “lockdown”, or (in its practitioners' perverse terminology) as “secure boot” — freedom 1 becomes an empty pretense rather than a practical reality. These binaries are not free software even if the source code they are compiled from is free.