Personally, I don't see any difference between both terms.
Easy. Some software is free (released in binary form), but not open source. So you get the binary and can use the program as much as you like, but
you don't have access to the source code.
Open Source projects are normally free and open. Meaning you can use it as much as you like for no cost,
and you have access to the source code.
Besides, if we could put it as simple as:
Free sw - anything under GPL license
Open Src sw - anything under LGPL license
No, no, no... you clearly don't understand the difference.
Would FPC be popular, if RTL were under GPL license?
No it wouldn't be popular at all. Why, because of the GPL license. If any program you wrote used that RTL, it would mean your application had to be GPL too (in other words - open source and GPL licensed). That is a GPL license requirement. So writing commercial software (closed source) with FPC would then become impossible - unless you implement your own RTL which is not based on GPL or on the RTL included with FPC. That would be a nightmare!
The Free Pascal team purposely chose the FPC compiler to be GPL, so the compiler itself is always free and open source. But the RTL was made available as LGPL with the static linking exception, which means it is open source and you have the option to create closed source projects.