No experience at Yahoo! here, but a quick look at the AltaVista page suggests the parent is correct.
The tabs eg. images, video, lead directly to subdomains of search.yahoo.com, so everything except web is handled very transparently on Yahoo's own pages.
I'm not disputing that there may be very little to modern AltaVista. But that's not really what I was originally referring to, either.
I'm not sure about Yahoo!'s policies, but most other large companies or organizations with custom software have processes in place to ensure the archival of old source code.
In these kinds of organizations, it's quite typical to have source code going back many years, if not many decades, for each release of a given software product.
This has gotten even easier as source control has become more widely used, especially during the 1980s and later, and as storage space has gotten far cheaper and larger. Many development groups have put great effort into ensuring that entire histories have been preserved when moving between version control systems, even when several such transitions have taken place. It can sometimes be possible to see change sets committed back in the 1980s, for instance.
Again, while I don't have any first-hand experience there, I'd be very surprised if Yahoo! did not have at least some of the early AltaVista source code archived in some way.
