May I ask exactly where you got this information from? Is it personal experience where you accidentally fried a hatchling? If it was from your research then I would expect a link to be provided.
I have 11 hatchlings on belly heat right now. 11 perfectly healthy "un-fried" hatchlings. I'll happy post some belly pics if need be.
My experience - personal hands on daily experience - is that belly heat is fine as long as it's controlled as all flexwatt must be anyways.