it does tell us that a sufficiently stupid, amateurish attempt to harvest human values using an infrahuman intelligence wouldn't reliably work.

You probably mean "reliably wouldn't work" :-)

However I have to question whether the Tay project was an attempt to harvest human values. As you mentioned, Tay lacks understanding of what she hears or says and so whatever it "learned" about humanity by listening to Twitter it would have been able to learn by straightforward statistical analysis of the corpus of text from Twitter.