FWIW I don’t find what you wrote confusing, but I do think it is following a sort of… bad convention that some companies have been pushing.

Opt-in and opt-out describe the nature of the choice that you make. “Opt” means to choose (apparently it is a French word we stole). Opt-in means you have to proactively choose to be in. Opt-out means you have to proactively choose to be out. “Opt-in by default” is an overly verbose way of saying “opt-out.”

In either case it describes the choice that you need to proactively make to override the default behavior.

Edit: I should also say that it is a “known point of contention” where pro-privacy people have been pushing back on this phrasing. So, you have probably accidentally stumbled into an ongoing discussion, which is why some of the comments might be unexpectedly prickly.