Don't worry, you're not stepping on toes. The thing is, most of the ideas in Wicca were borrowed from other sources. Especially Wiccan religious beliefs. Wicca is an attempt at reviving the "old religion". To do that, Wiccans had to revive the old beliefs of the old religion. But there are MANY "old religions" (Egyptian, Greek, Celtic, African, just to name a few), so not only Wiccan beliefs in general eclectic, but often the individual beliefs that one Wiccan adopts contradict the beliefs that another Wiccan adopts. For example, one Wiccan may say there is a God and a Goddess. Another may say there are many gods and goddesses. Another may say there is only one god, the Great Mother. One may say the God was born on Yule (Dec 21), another that he was born on Imbolc (February 2nd). One may say that the alter should go in the Northern Quarter, another that it should go in the Eastern Quarter, another in the North-Eastern Quarter, and another in the very Center of the Circle. One Wiccan may say sabbats must be done skyclad (naked), another may say it is okay to wear clothes, as long as they are ceremonial (like robes), and another may say that practicioners can wear whatever they want, that it is not important.
Also, with a revivial of the "old religion", two things may happen: the original people working to "revive" the old religion may know WHY a thing is practiced, but modern practicioners may not. The original people may know where an idea came from, while modern practicioners may not. Many Wiccans believe whatever tenants of Wicca that they want without knowing much about the origins or significance of their beliefs. My mother has been practicing Wicca her entire life (she was taught Wicca by HER mother), and yet she does not study the origins or the "why" of her beliefs.
Your friend is wrong. If you read the translations of myths about the Tuatha De Danann as they were recorded by St Ciaran, you will find that Tir na nOg is definitely not some modern invention. Even if it was not found in the oral myths of Celts back in ancient times, at the very most it was created around 650 CE, not in modern times. The WORD Summerland is a modern invention, to which modern pagans (Druids also believe in Summerland) have applied old ideas. Two things stand as truth: the ancient Celts believed that the soul, after death, went to Tir na nOg to rest and await rebirth. Modern pagans have adopted a similar idea, and invented a word to describe it that is very similar to the english translation of Tir na nOg, "Land of Youth".