“In computer science, a deterministic algorithm is an algorithm which, given a particular input, will always produce the same output, with the underlying machine always passing through the same sequence of states”
In turn what the author is saying is precisely what he says, no more, no less - explicit code is code whose behavior you can understand just by reading it, with no external context, like how the runtime currently works on a particular hardware.
The examples he gives are very clear as well. For example he claims to know the stack allocations that will happen just by looking at the definitions of the data types. You can’t know this on the JVM for example because the JVM does optimizations under the hood that the programmer cannot control or reason about. This doesn’t yield non-determinism. What happens is that the developer ends up programming for a higher level machine that hides some details of the underlying hardware.
Explicit isn’t always better than implicit, an argument that the author also makes.
Overall I find the article very compelling and I’ll be sure to use it for reference, as it’s what I also think, but with better words.