AVI is just a container format, it can hold video that's been encoded with any one of a number of codecs. So, if the video uses DivX as its codec, then Quicktime decodes it with DivX. If the AVI file held some other format, you'd need a different decoder. If the AVI had DivX and you didn't have DivX installed, it wouldn't play.
That is a REALLY old version of DivX though. You might want to go to labs.divx.com and get the latest DivX beta for OS X.