I think you will find that all that does is use an earlier set of install scripts. Not actually an earlier Alpha version.
The alphaX.tar.gz files are just the install scripts that trigger the download of the debs, set up the system, etc. But the debs themselves are always the latest Alpha version, even if you use an earlier script. In fact, as the alphas and the install scripts are changing over time, you will probably find that really early versions of the install scripts will fail to install and setup the later debs properly.
In short, don't confuse the install scripts versions with the actual Alpha versions, they are not the same thing. And you can see this from looking at the date stamps on that link... the "latest" is May 15, which is far older than the actual current Alpha... this is because the install process has stablised somewhat, now only the content of the debs is changing.