There is a big difference between shipping a battery Vs personally having it with you on a plane!
iFixit needs to comply with shipping rules, hence they can’t ship batteries via air. They need to mark the package declaring it must not travel by air freight (DHL, FedEx, UPS or USPS/foreign postal services),
As far as what you do hand carrying the battery onboard, you can without any problems! As you’ve noted its well within the limits of US and IATA rules.
But! You will want to remove the labels from the box so you don’t scare anyone! Frankly, I would get a thick poly bag which you can put the battery into so you don’t waste the space given what else you will be carrying if you are leaving it bare!