All I know is what I can see from the X-Spam headers they add with that matched rules, which are SA's. FastMail has been around for ages, and I guess it's "good enough" (or at least, not "bad enough" to replace). It's not so easy to replace a tested (though imperfect) solution like SA with some new AI stuff, even of it would be better; it's all about ROI.

Also, not sure if Howard is still involved to FastMail? They sold to Opera in 2010, although they've been independent again since 2013, but it looks like the 2010 sale was the end of Howard's involvement?