The Windows Insider builds are available to anyone who can be bothered signing up to the program. The only 'leak' here is if publishing screenshots constitutes a breach of the EULA.
Microsoft only releases some builds to the insider program. More to the fast ring than the slow, but they certainly do not release every internal build to the insider program. For example, the currently available build for PC is Build 10130. For me, it's always interesting to see what's in the latest internal builds even if they aren't released to the public - just to know what to expect when a build is released.
Also, this is not just a screenshot release. The entire build was leaked, and this is the appropriate word, because it is not available through official Microsoft channels. The story chosen for the summary was just one that shows a lot of pictures primarily in place of writing more informative content.