It's not. The US Constitution is clear - Article 1 Section 8 gives Congress the "Power To lay and collect Taxes, Duties, Imposts and Excises".

The President is given no such authority.

Trump's argument was that Congress did authorize him to implement reciprocal tariffs - some other country taxes our goods at 30% so we tax their goods at 30%. The problem is most countries don't tax American goods very highly, so Trump has argued that our trade deficit indirectly proves there are non-tariff barriers (like extra regulation for American goods). That argument is unlikely to prevail in courts over the long term - for one thing, non-tariff barriers are not tariffs.