You have made the main point here. These DNA tests are trash, money-makers. Their claims go way beyond what they actually do prove. Suckers are born every minute and charlatans feed on them.
No, they aren’t trash. They match you with relatives you didn’t know who’ve also taken the test. They are very good at that.
Where they sometimes fail is what dna they assign to certain ethnic groups. They match you with ethnic groups, not countries of origin - sometimes they differ.