Whats the source you have for this information?
My own experience. I coded async DHTML back in 2002, with iframes or async XML loading (since 2005 named AJAX). I debugged GMail back then (the first version with the blue rounded corners, the same design as Google Groups and Google Code)
It's incredible hard to find older web resources from 10 to 15 years ago, as most bookmarks don't work anymore. And some current websites sadly try to rewrite the history.
e.g. Aaron mentioned it: http://www.aaronsw.com/weblog/ajaxhistory and you can search through Slashdot.org, (former) digg.com and web.archive.org
Both GMail (2004) and Google Maps (2005) used iframes for the first years. There were still many users with older browsers without real AJAX support - Firebird/Firefox/Mozilla Suite/Netscape, Safari(?) and some Opera. Only IE 4/5+ and current Firefox/Mozilla browser supported real "AJAX" in late 2004. Iframes were supported by most browsers.
GMail made async HTML popular and well known. AJAX got popular in 2005 around the same time. Many media articles often cited GMail as a good AJAX website back than. The reality was, GMail used invisible iframes for async data transfer and not AJAX. One reason was browser backwards compatibility. Memory leaks were an serious issue in early GMail days, especially IE 6 users complainted a lot. Gmail switched to AJAX around 2007. Interesting is also that AJAX was added to IE 4/5 for OWA (outlook web access) in ~1999 but it was little known and got popular as Mozilla completed the API reimplementation in 2004/5.