These moves fairly strongly suggest that build 17133 *is* going to be released as the long-awaited (and overdue) "Creators Spring Update", otherwise known as Windows 10, version 1803.

UPDATE: It turns out Microsoft were intending for build 17133 to be the Spring Creators Update, but they discovered some issues which could lead to a "higher instance of blue screens (BSOD) on PCs". As a result, they've now pushed build 17134 to insiders in the Fast ring for testing.