In this letter, Dijkstra talks about readability and maintainability in a time where those topics were rarely talked about (1968). This letter was one of the main causes why modern programmers don’t have to trouble themselves with goto statements. Older languages like Java and C# still have a (discouraged) goto statement, because they (mindlessly) copied it from C, which (mindlessly) copied it from Assembly, but more modern languages like Swift and Kotlin don’t even have a goto statement anymore.
I wasn’t saying that unit tests replaces readability, I was saying that back in the 60s they’d reason and debug using their brains (and maybe pen and paper), with more use of things like formal proofs for correctness. Now that we write more complicated programs in more powerful environments, it’s rare to do this (we’d use breakpoints, unit tests, fuzzing, etc).