> Isn’t something like the MIT License open source but not “free software” in the rms sense because it does not encumber people who build on it to in turn produce free software themselves?
No, the FSF specifically IDENTIFIES MIT as meeting the Free Software Definition, which does not require any such requirement.
Now, Stallman and the FSF have identified reasons that they prefer copyleft licenses like the (A)GPL for many uses, but that isn't the same as considering them the only Free licenses.
> The open source page also says “ The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.” which would seem to disqualify the GPL.
The GPL explicitly does not put any restrictions on other software merely because if is distributed along with the licensed software.
No, the FSF specifically IDENTIFIES MIT as meeting the Free Software Definition, which does not require any such requirement.
Now, Stallman and the FSF have identified reasons that they prefer copyleft licenses like the (A)GPL for many uses, but that isn't the same as considering them the only Free licenses.
> The open source page also says “ The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.” which would seem to disqualify the GPL.
The GPL explicitly does not put any restrictions on other software merely because if is distributed along with the licensed software.