Two strategies I was thinking of were technical peer reviews and formal proofs. Of course, these aren't mutually exclusive either with each other or with automated testing, and these are all generic terms that each cover a multitude of specific implementations.
All three have a strong track record of finding bugs when implemented well. All three also add significant overheads, so there is a cost/benefit ratio to be determined. The relative costs of implementing each strategy will surely vary a lot depending on the nature of any given project. The benefit for any of them would likely be significant for a project that didn't have robust quality controls in place, but you'd get diminishing returns using more than one at once.
I could easily believe that skilled developers had evaluated their options and determined that for their project some other strategy or combination of strategies provided good results without routine use of automated testing and that the additional overhead of adding the automation as well wasn't justified.
All three have a strong track record of finding bugs when implemented well. All three also add significant overheads, so there is a cost/benefit ratio to be determined. The relative costs of implementing each strategy will surely vary a lot depending on the nature of any given project. The benefit for any of them would likely be significant for a project that didn't have robust quality controls in place, but you'd get diminishing returns using more than one at once.
I could easily believe that skilled developers had evaluated their options and determined that for their project some other strategy or combination of strategies provided good results without routine use of automated testing and that the additional overhead of adding the automation as well wasn't justified.