The difficulty isn't normalizing (per square foot is probably the most reasonable), it's getting death information for individual buildings. Outside of the most famous buildings it's not easy to track down.
In general, worker safety has improved significantly in the 90 years since the Empire State Building was built, see here: Workplace Fatalities Fell 95% in the 20th Century. Who Deserves the Credit? - Foundation for Economic Education (fee.org)
It's pretty common to interpret slower speed as the inevitable cost of increased safety, but looking at some notable projects the link is less than obvious to me:
-5 workers died during the construction of the Empire State Building, which was built in 11 months
-0(!) workers died during the construction of the Chrysler Building, which was built in 20 months.
-5 workers died during the construction of the Sears Tower, which was built in 4 years.
-2 workers died during the construction of One World Trade, which was built in 7 years
-60(!) workers died during the construction of the original World Trade Center, which was also built in 7 years.
It would be interesting to do a more thorough analysis, scaled to building size, but it's not trivial to do (I did a quick check for number of deaths on some less notable buildings and they're much harder to find if they exist at all). It's a reasonable hypothesis, but most people suggesting I think are going off vibes rather than actual data. And it seems clear that it's at least in-principle possible to build both quickly and safely (though you could make like, a stochastic argument against this).
Re: other examples - true interchangeable parts, which was a major manufacturing advance, required a lot of advances in precision manufacturing. It had been attempted as early as the early 1700s, and was made much more feasible/cost effective by the invention of high-speed tool steel in the late 1800s, which made it possible to machine heat-treated parts. Interchangeable parts was, among other things, one of the technologies that made Ford's assembly line possible (iirc, Ford was the very first car manufacturer to use interchangeable parts.) But as late as the 1940s, it was still expensive to get true interchangeability, and wasn't always used.
Re: portable brick machine, I think automation would have to advance a lot before something like this ended up being cost effective (and the resulting automation could probably do a lot of other, more interesting things than just "assemble bricks")
Re: mortar, folks are already doing this (this is what fastbrick robotics uses, basically)
Re: 3D printing, I don't think this is especially likely, mostly because people want brick specifically because of how it looks - it's already a sort of cost-inefficient system that people choose for the aesthetics. Adding another system to the mix doesn't seem like it would change this calculus, even in the event it becomes super efficient (which seems unlikely to me).
For folks interested in this topic, I found "Air Conditioning America" a good complement to this book. It covers a narrower period of time (from 1900 to 1960), and it's more academic, but it goes into quite a bit more depth than "Cool" does.