Your first sentence is wrong for many scenarios and is meaningless without a time interval. Over a period of 10 years the risk historically is absolutely higher from inflation than a total market mutual fund.
If people were good at sweeping excess balances into their investments I think this idea would be less appealing. But for a variety of reasons, many perfectly rational, people like having a nice cushion in there. It feels safe. Long term, it isn’t. These guys should lean into that in their marketing.