As far as we know it's true. I for one am not going to tell the Principia author that he's wrong and index it against his will. See KSP-CKAN/NetKAN#6242 for the last time this was asked.

The thing is, maybe it also stops mods other than Principia from integrating CKAN even though they may agree if it was technically possible. this is not an issue related only to Principia, please reopen or close for good reason as my second question wasn't answered

As I understand it, their concern is that CKAN can't forcibly install vc_redist on the user's computer. I would not anticipate that being added.

EDIT: Confirmed, Principia's wiki states that Windows users (may) need to install vc_redist, and Linux users need libc++:

https://github.com/mockingbirdnest/Principia/wiki/Installing,-reporting-bugs,-and-frequently-asked-questions#its-not-starting

Well the generic solution here would be to enable custom install scripts

Or for anyone who wants to run Principia to install it outside CKAN, because CKAN doesn't change anything outside the KSP directory and its own entries in the registry.

Again, this issue is not related specifically to Principia but to any mod that may need external dependencies