SIPC guarantees that the securities held in your account at a brokerage will not be lost if the brokerage fails; it does not guarantee the value of those securities. See https://www.sipc.org/for-investors/what-sipc-protects .
Regarding money market funds generally and their (usually very low) risk of “breaking the buck”, see https://www.investopedia.com/articles/mutualfund/08/money-market-break-buck.asp and https://www.investopedia.com/investing/do-money-market-funds-pay/ . Note: the referenced US Treasury guarantee program for money market funds existed only for some time in 2008-2009.