By harming do you mean on price? Big tech has the potential to harm through externalities (privacy, control of the media, etc.). A sufficiently free market - with appropriate safeguards to ensure competition and prevent regulatory capture - tends to deal with price just fine, but not with externalities.

Warren's argument here is a straightforward free market one: free markets only work when there's a free flow of information. If Amazon both runs the market and participates in it, it's an inherently unfair playing field. Her argument is not that we should break up companies that we don't like or even those that hurt consumers; her argument is we should break up companies whose intact existence threatens the fundamental nature of free markets.

All that said, as it happens, she opposed Comcast's attempt to buy Fox https://thehill.com/policy/technology/396559-senate-dems-urg... and the T-Mobile / Sprint merger https://thehill.com/policy/technology/429618-dem-senators-ur... — opposing both on the grounds that they're anticompetitive.