There is really nothing illogical about this, autoconf's configure.ac is written in ... you guessed it, autoconf. So you need autoconf to generate it. What would be illogical would be to write configure.ac in something else, when we already have autoconf.
As a user, you do not need autoconf at all. As a developer of a project that uses autoconf, you don't need autoconf from git. But as a autoconf developer, you will need a recent version, since it would make little sense not to. Hope that helps.
