So some applications don't have a plugin interface at all?
Right. They have no plug-in interface. Most don't, in fact. I mean, that's something a programmer adds if there is a need, not by default.
However, the poster was expressing the possibility that with *MUCH WORK* an external programmer could come in and create such an interface. Without the source code, this requires massive amounts of work, reverse engineering, and even then it may have limitations or be imperfect. The mechanism would involve injection into the process, then modification of the applicable code, on the instruction set level -- making the whole thing also very prone to errors and other risks.
In the case of signed executables, other platforms, or certain protection schemes, creation of such a 'third-party API interface into a program that has no plug-in interface' could be infeasible.