I really can't go through the whole history thing--it is outlined at the site I mentioned above, but there does not seem to have been an extended period of time when it would have been necessary to conceal the faith--
There certainly may have been tradtional garden figures that had religious meanings associated with them--as is the case with much that is traditional, the original meanings are long forgotten--
However, the problem we have here is that none of us knows what the book actually had to say--whether it quoted real text, or, as is often the case, it made a passing mention of an otherwise unexplored facet of gardening--
As an occasional student of the history of the occult, I have heard and read all sorts of claims about secret meaning of signs and figures, hidden messages, and, of course, related political intrigues.
Almost anything could be true, evidence for these sort of things is often weak--that is, it doen't really quite say all that it's presenters say it does--their response is that everything was secret and clandestine, so that there wouldn't be any really substantive evidence--but of course this means that there is no way of telling what is really true--
I find it unfortunate that the burden of proof is always on the person who wants to withold judgement--as in the case of this "12 Days" business, the assumption seems to be that the people who presented it (who ever they were) had to be accepted at face value until somebody (or some group of people) debunked every aspect, no matter how unreasonable that it was--
As far as I can see, there is not one bit of evidence to support the truth of the "12 days" thing--and here we are having to run in circles doing detailed histories of the relations between Rome and the C of E--and it is never enough, because all someone needs to to is say, "I saw something somewhere" and not even be specific about what or where, and we are all back at square one again--