Even without truth social, his value was basically only on paper... papers he has repeatedly and for a very long time lied about. If he sold off all his properties, would he be rich? Multiple hundreds of millions, sure. Billionaire? Maybe.
No one will buy his properties from him at his asking prices, it's already known he vastly and ridiculously inflates them. Legitimate banks won't deal with him: he's been found guilty of large scale, long term fraud against the banks AND the federal government (which is an aspect the world and the US government seems to be largely ignoring.).
The forbes list operates largely on good faith, and publicly sourced information. They're not hiring forensic auditors and pouring over the intimate accounting details. Trump lied his way into everything, why not lie his way onto the forbes list?