I wouldn't say Foster corroborated anything. He just copied it.
He started out doing serious work on his own ancestors, but there wasn't any money in that. He made a living out of being a human Xerox.
He's been described as totally uncritical, but that's not fair. Modern critical genealogy didn't get started until later. Most fakes hadn't yet been recognized and debunked, and Foster wasn't the man to do that. But he wasn't particularly credulous by the standards of his time, and he wasn't Burke, he didn't cynically republish known and obvious junk.