Were you involved with this process? I'm the author of the blog post, and I'm looking for people to discuss a rewrite of that POSIX pax page. What bothers me most about that section is that it prescribes that if you don't support a (vendor) typeflag, you should treat it as a header for a regular file, extract it, and continue with the next header. The problem is that vendor extensions may change the framing of the archive, and POSIX sets a precedent for that with pax headers, where you can override size=... of the next header. It should just drop that, or say that the implementation may error. There are more such improvements to be made.