Well, your post pretty much points out that a lot of coaches have little to no training or expertise in talking about or understanding of how body weight relates to performance but yet it is consistently used in discussion with athletes across genders.
First off if this accurate an athlete who is 5'7 and 107 or 98 pounds is way underweight so a coach saying they are fit is...wrong and probably should stfu and was most likely guiding the rest of you wrong too despite your own impressions or interpretations.
Coaches seem to use weight as an easy talking point to improve performance, but its a red herring and this silly post overall is a red herring.
If a woman is struggling in her running, then guess what male coach, its time to look at the constellation of factors and take responsibility including examining if your training system has flaws, is communication breaking down, is there a trust issue, if there is a diet issue or confidence issue get them to the proper people.
The reality is there are a lot of really poorly trained coaches out there with basically no credentials except experience in running telling women they "look fit" based on nothing more than their own eyeballs and anytime they are called to task for their mediocrity they wither up and write posts like this saying they want to stop coaching women BC its hard.