I've got to believe a DOT helmet at least provides some level of protection.
That's the rub. A Helmet that meets DOT standards does provide some level of protection. The crux is that DOT doesn't check if the MFG actually does meet specifications. DOT is given a sample helmet that is tested. They then can display DOT on the helmet. MFG can self certify and self test though. see her:
Helmet Performance Testing
There is a 40% Failure rate when actually testing DOT Certified Helmets.
My take is a reputable MFG that is DOT certified is probably actually certified and will give you the protection you are paying for. There are certain Asian Countries that I would not trust to provide either a genuine DOT certified Helmet, or send a GOOD Sample to DOT and then sell inferior products using that certification.
Case in point:
You think this Helmet for $4.10, reduced from $12.82 with a DOT Sticker on it will offer the protection of a genuine ECE or Snell rated helmet?
If you search YEMA helmet on AliExpress, it isn't DOT certified on that site. DOT stickers are cheap.
NHTSA here:
NHTSA Helmet Testing List tested helmets. Unfortunately is only up to 2022. No Yema Helmet in 2022. So it might have actually passed. Or, they might just claim it and throw stickers on.
Just make sure what you are buying is legit. Just cause Amazon says it, doesn't mean it's true. As Abraham Lincoln once said, "Don't believe everything you read on the internet".