You are babbling full-on gibberish now.
"The people who are a party to shared source licenses, have access to the source"
So do the employees of the original vendor. So what? How does that matter at all? It means nothing and has no bearing on this topic.
"FOSS software can also be subject to NDA."
Bwhahh wut? No. And I mean, by definition, no.
An organization may take some foss and use it internally and hide that modified internal version, but that means nothing to anyone else. They can't nda the original software they forked from, and they can't even nda their modified version unless the original happens to be MIT-like or they never redistribute it.
There is a lot of MIT software which doesn't require sharing back, but that still doesn't change anything.
Their private mods are no different than anything else they write privately. Whether the private thing is a diff off of a public thing, or something standalone, makes no difference and has no bearing on the original public thing. Their new private thing, if it is nda'd, is then by definition, not foss. It doesn't matter that it's comprised of a copy of some other foss plus whatever they added. It's now just some proprietary software like any other.
None of that harms anyone else or makes foss somehow just as limited as proprietary or proprietary just as available as foss.
What a bizzare claim to try to make.