That depends on the circumstances. It's not at all unusual (though becoming rarer) for a brown-, purple-, or even blue-belt in BJJ to teach. It started out that way because there simply weren't enough black-belt folks available in many areas. It continues mostly because it worked before. And it also depends on how people are educated about what the rank means. Where BB is sanctified, you're more likely to get that kind of reaction where if it didn't come from a BB, it's probably not right.