Philosophy of science is less about telling scientists how to behave than trying to understand what actually happens when scientists do what they do, what science accomplishes on a theoretical level, what the difference between science and non-science is, etc.

It's less of "here's how to get to the truth" and more of "can science get at the truth at all?" or "to what degree can science get at the truth?" or (much like Feynman's essay) "what makes these particular actions or beliefs 'scientific'?"

Karl Popper's criterion of "falsifiability"[1] was once a popular one, in regards to what makes something 'scientific'. But that's fallen out of fashion[2]:

"Sir Karl Popper is not really a participant in the contemporary professional philosophical dialogue; quite the contrary, he has ruined that dialogue. If he is on the right track, then the majority of professional philosophers the world over have wasted or are wasting their intellectual careers. The gulf between Popper's way of doing philosophy and that of the bulk of contemporary professional philosophers is as great as that between astronomy and astrology."

And so the debate continues...

[1] - https://en.wikipedia.org/wiki/Falsifiability

[2] - https://en.wikipedia.org/wiki/Falsifiability#Criticisms

That doesn't really mean Popper is wrong.

If it can't then by its own measure, it's not "scientific". Further, if one scorns anything "unscientific" as valueless, then the principle of falsifiability must also be valueless.

In the relevant sense, yes. We switch out conceptual definitions when they fail to encompass the relevant empirical instances. If falsifiability turned out to allow, for example, theology to be considered a science, then we would have to throw out falsifiability. As it is, it's only a coarse-grained principle for demarcation, hence the arguments about string theory.

I think that may have been Feynman's point.

Whether, or to what extent, Popper's falsifiability criterion is a good description of science is not a problem for scientists.