I think the conclusion is correct, though the test is not done properly ;) — the function name argument to prototype should've been prefixed with CORE::.

Compare the results using a function that is overrideable, like open

for my $func qw(CORE::open open) {
    my $p = prototype $func;
    print "$func: ", defined $p ? $p : "(not overrideable)", "\n";
}
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Output:

CORE::open: *;$@
open: (not overrideable)
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