They are the same thing!
One comes from Latin (likely through the Normans AKA French):
avocare: to call away/from
"ab" (away) + "vocare" (to call)
in plain English: The mountain is calling! (as in, work is done, time to ski!)
the other hobby from the name of a farm horse (with which you ride during one's leisure time)
Because French was spoken by the aristocrats in England during the Norman Conquest, words that have been adopted from French typically have a higher register than words of other origins. The register ranking:
- Latin (language of academics)
- French (language of aristocrats during a critical period in English history)
- Germanic (language of the Anglo-Saxons AKA most people at the time)
- Other (other adopted words)
What I think Janus Bahs Jacquet means when he stated that avocation is "bookish", is that it is a higher register word (used in formal situations).