Not exactly equivelant. A swastika is an image that has no non-racist symbology. The southern flag is a symbol of southern pride. While it has been used as a racist symbol as well, no doubt, having legitimate non-racist symbolism means any sort of an outright ban from a search engine would be absurd.
That said, I'm not a fan if google banned that either (or any searchable content that isnt illegal like CSAM or something). But these arent equivalent comparisons.
Also not being able to find something in a search is not the same as censoring it. There are a lot of things i cant find with duck duck go that is clearly not censored. Where exactly did you find evidence suggesting duck duck go was explicitly censoring content to eliminate southern flags?