[anm89]

People refer to this as the greater fool theory. Softbank never believed that valuation. What they believed was that there was someone dumber than them that they could trick into buying it from them leaving themselves with cash and the other person with the overvalued asset.

In this case it turned out they were the first and final fool and they were left holding their own bag.

And thus they essentially sacammed themselves(or really their investors) out of billions.

You'll also hear this referred to as a pump and dump. Pump up the value of some asset and then dump it on someone else.