This is not really a Safari-only thing. All password managers that I have used in the past had some kind of heuristic to decide whether a field should be auto-filled or not. Here is a nice explanation by a (former?) 1Password employee (https://1password.community/discussion/94198/autocomplete-of...).
To me as a web developer (among other things :D) this is quite annoying because password managers often hijack our forms when they decide that the label (or id or classname etc.) sounds suspiciously usernamely, passwordly or credit cardly.
To me as a web developer (among other things :D) this is quite annoying because password managers often hijack our forms when they decide that the label (or id or classname etc.) sounds suspiciously usernamely, passwordly or credit cardly.