If you believe their attestations, they pretty much are. See, for example, https://assets.ctfassets.net/vyse88cgwfbl/2SGAAXnsb1wKByIzkh.... Most of the money is in Treasuries, or markets like overnight repos that behave much like Treasuries. Most of the rest is in things like corporate bonds, which again generally move like cash. Only their claimed excess equity is in riskier investments, like Bitcoin.

If you doubt their attestation, it is more reasonable to doubt their claimed total assets before worrying about the breakdown of their investments.