A good question. When I wrote that, I was playing mostly Deity level, 7 civs, with no concern for score, just time. My thinking was - You probably want to make lots of vet crusaders (30?) just in time to use them, eg near the end of the game. It's more efficient to build Sun Tzu than 8-10 barracks because it costs fewer shields, and because you can build it in your homeland (where the shields are less valuable; your more distant cities will build the crusaders). It's usually faster that way, too.
But I don't build SunTzu very often anymore. I play mainly GOTMs, usually at lower difficulty levels. I can often conquer with only about 15 crusaders, made from approx 4 cities with barracks, and it isn't even crucial that they are all vets. So, SunTzu isn't really needed. I don't like having Feudalism, mainly because the AI always seems to get it from me. Then my crusaders have to face pikemen, which changes the combat odds quite a bit.
Also, I play a more relaxed game now, with some concern for score. I trade more now, and often have surplus gold for RBs. And I don't mind waiting for a few barracks to steadily produce a decent vet army.
Rush-buying costs more after Feudalism, but this bothers me less than the pikemen. I guess it adds about 10 percent to the cost of a typical unit. Not good, but not a disaster.
So, IMO Feudalism+SunTzu is a reasonable option in EC games, but it isn't necessary. It probably makes more sense in 7civ/Deity games than in most of our GOTMs.