To your specific point: "hardly any engineers quit NASA to join ISRO", they do not need to. Many of the underlying technologies come from a variety of sources.
All you need is a few to be the mentors or lead. ISRO has been around for a long time and so they do have a depth of their own experiences, plus what ever talent is developed locally or by folks who learn from others in stints in Europe, USA, etc. ISRO has also lots of experience since 1950s? It would be interesting to learn how much COTS stuff went into their MOM (% wise...)
AFAIK,ISRO folks who do share whatever is possible and makes sense. I have worked with a few ISRO folks and they have shared and published whatever they can.
I totally agree 100% with the rest of your observations.