Isn't Wicca a new religion?

Isn't Wicca a new religion? 

Yes and no. It is a little complicated. As far as we can definitely prove it is a new religion. 
I have Triumph of the Moon by Prof Ronald Hutton and as an academic book it can only speak to the sources it can prove. 

Much of Western mysticism's knowledge and beliefs were passed on orally (especially in cultures with strong oral traditions like Ireland, Cornwall, Scotland, parts of Spain and Wales). While "Old Dot" was never seen as a serious person to academics many non-academics believe she did exist and did run the coven Gardener joined. While his flare for the dramatic and study of rituals from all over the world influenced his rites and practices this person seems to have existed (read the essay chapters in The Witches Bible Janet and Stewart Farrar).

 While Dorothy's "wicca" was undoubtedly different to Gardeners (if you read his book of Shadows you can see edits of important texts deliberately left out to ensure that the oral tradition was maintained. One of the reasons Wicca feels ancient is because in ritual context time is so different. Ritual being the core of these practices and being a mutable and powerful force to change not only those within the circle but outside, it is hard to express this sensation of time, of "before", of echoes of something without using the world "ancient". Ritual (Victor Turner) is at the core of social, mental and spiritual changes. Ritual work dwells in a place of symbolic, of archetype, the sub-conscious and for practitioners, the spiritual and Divine. These places are ancient. They are as old as symbols and just as full of paradox, depth and opposing forces. If at least once a month you dwell within this space within the ritual circle and yourself it is hard to dismiss this sensation of being in the presence of something very, very old.