There is no scientific evidence for any of this. Signs of physical habitation have never been found near Borth.
I really wish people wouldn't abuse the word scientific this way. "Researchers have found no relics of human habitation near Borth" would have been a lot more accurate. Who knows what will turn up next Thursday? And failing to find something on high ground doesn't mean there isn't something a little farther out, now underwater. Absence of evidence is not evidence of absence. Remember that silly report earlier this month that allegedly "rocked the Bible" because they hadn't found camel bits until O(1000BC) in Israel? Camels were known to have been in use in the East 1500 years before that, and the Genesis references are to people either from or with connections to Mesopotamia. Sheesh.
A walkway made of sticks and branches was also discovered. It's 3,000 to 4,000 years old and was built, it is believed, to cope with rising sea levels back then.
If I were to bet I'd bet that the forest, or something like it, inspired the stories, but wasn't the source. Though some literature is very old, and probably some stories are older.
However the legends ran, the place looks fun to visit.