I think a "fly" is a butterfly? So a spread against another spread, but may be wrong. He does mention march/june vs sep/dec so that makes sense.
This too interests me, have always been interested in learning more about spread trading, mainly why they do it, and not just trade an outright.
I think the main problem is getting the right fills at the right spots, those charts where it looks very nice to fade the extremes back to the mean are actually pretty hard to get into at those extremes because usually it's a pretty liquid contract and it can change the chart if price just touches an area, like it might hit the offer which would then make the spread chart have the extreme spike, but you can't actually enter there because the order book is so thick and you can't get filled....unless you put orders in ASAP on the open, which I know is what a lot of aussie bond traders do. I guess they have figured out where they want entries and place orders accordingly to get a good position in the queue.
But again just my assumptions, I'm like Jon Snow, I know nothing.