Ah, upon rereading it turns out it is still an issue: ... would fail to run test commands whose left-hand side matched a unary operator. This was fixed [...]

Ah, upon rereading it turns out it is still an issue: ... would fail to run test commands whose left-hand side matched a unary operator. This was fixed [...] in 1979. However, test and [ were also available as separate executables, and appear to have retained a variant of the buggy behavior: