I'm not sure where the idea came from, but it probably is a "meme" current among Christian qabbalists and not among Jewish qabbalists fluent in rabbinic Hebrew. It would be strange for someone knowledgeable of Hebrew to not realize that the name derives from the root ys' (yod - shin - 'ayin) "to save"; yhws' means "Yahweh is salvation".
The original spelling that I gave in Joshua, yhws' (pronounced Yehoshu'a), then underwent change in post-exilic Hebrew. Thus in Nehemiah 8:17 and Zechariah, we find the name shortened to yws' (or Yoshu'a), with the medial he deleted. Then, in the Dead Sea Scrolls from the first century BC and the first century AD, the name is shortened again to ys' (or Yeshu'a), a development that obliterated the original theophoric element and made the name superficially identical to the verbal root ys' (cf. 1QH 15:15, fr. 18:5). Throughout the whole process, the 'ayin formed part of the name. The form yhswh never existed in Hebrew, at least as a version of "Joshua" or "Jesus," and it looks like a naive error made by those unfamiliar with Hebrew.