First, that is not the only example of penance I gave, and I could give more.
Second, the baptism of St. John, as well as the Christian baptism, is linked to metanoia. In nearly all examples of baptism in the Bible they occur together.
If you wish to argue that what St. John did had no relationship to what he preached, — maybe he liked camel hair! — well, go ahead and argue that. As Catholic, I’ll stick to what the Bible is telling me, and I don’t think the acts of penance done by St. John are there by accident.