Index-based robo-advisors generally invest your money in vanguard, ishares, schwab broad-market etfs, which come with their own fees (industry-lowest). The expense ratio is the accumulation of all those fees. To check the accuracy of the claim, you would need to find the specific instruments each company invests in, at what proportions, and add up their fees.

Cash drag is the penalty you pay for the time and amount of your wealth that is spent in sub-productive, inflationary cash. The article states:

"Schwab allocates up to 30% of a portfolio to cash. In certain circumstances, keeping up to 30% in uninvested cash can result in up to a 0.56% annual return penalty"

This sounds like a worst case scenario. To roughly calculate cash drag, you can take the avg percentage of wealth that will be in cash throughout the year, then multiply by 5% rule of thumb avg returns. For example if you had to keep 10% in cash, that would be 0.1 * 0.05 = 0.5% in lost potential earnings due to cash.