Earlier quoted context omitted.
Ignorance leading to assumptions. Their eureka moment: "The shell, not the Linux kernel, is responsible for searching for executables in PATH!" makes it obvious they haven't read up on operating systems. Shame because you should know how the machine works to understand what is happening in your computer. I always recommend reading Operating Systems: Three Easy Pieces. https://pages.cs.wisc.edu/~remzi/OSTEP/
The thing is, though, that PATH being a userspace concept is a contingent detail, an accident of history, not something inherent to the concept of an operating system. You can imagine a kernel that does path searches. Why not? There's a difference between something being a certain way because it has to be that way in order to implement the semantics of the system (e.g. interrupt handlers being a privilege transition)…
Yes it does, but the more surprising thing is (coming from AmigaOS with its dos.library function ReadArgs()) that the shell does this. The shell is also responsible for argument expansion - madness!
On AmigaOS, when you type "delete foo#? force", the shell passes the entire command line to the delete command. The delete command calls ReadArgs() with a template (FILE/M/A, ALL/S, QUlET/S, FORCE/S), and the standard OS function parses it into lists of files, flags, keyword arguments, etc. The "file" passed is "foo#?", and the command uses MatchFirst()/MatchNext() to do file pattern matching.
Every command (that uses ReadArgs() and didn't plump for "standard C" parsing) has the same behaviour: running the command with "?" gives you the template, which tells you how to use it. Args are parsed consistently across all programs.
Then you get "standard C", which because K&R and main(), ignores this standard Amiga parsing function and just does naive splits. Across multiple Amiga C compilers, quoting rules are inconsistent. Amiga C compilers have to produce an executable, and it knows it'll be called with a full command line, so the executable itself has to break that into words before it can call main(), and it's up to each compiler writer how they're going to do that. Urgh.
In unix-land, it's up to the shell to parse the command line, and pass only the words... hence why the shell naturally does all the filename globbing, and why you have gotchas like when these two commands are sometimes the same and sometimes they're not:
find . -name foo*
find . -name 'foo*'