I wasn't involved in the original discussion, but curious about your question, so did some research and found some advice offered by the FeedXL nutritionist Nerida Richards. Apparently eggs contain a protein called Avidin in the egg white which binds biotin and stops the horse from absorbing it. She also says
"While I think you would have to feed a lot of raw eggs to a horse to produce a biotin deficiency my usual response to whether or not horses should be fed raw eggs is that they provide little benefit (an egg being so small and a horse being SO big) and will reduce biotin absorption to some extent, so it probably isn't a good idea. Depending on why you are feeding them (ie was it for the oils, the protein or the energy?) you will be able to find an equally effective plant source to fill those gaps."