At the time I had an HTC TyTN 2 phone that did everything the iPhone could do and more, and had a removable battery, about the same size as the battery inside the iPhone 1. If you look at photos of the TyTN 2 battery, and the iPhone 1 battery, they are very close to the same size.

There was very little difference between those batteries. The TyTN 2 had a 1350mA hour lithium battery. The iPhone 1 had a 1400mA hour lithium battery. Practically no difference there in size or capacity.

So, no, the iPhone's battery was not a "technical marvel" any more than other phones of that era.

And in fact Apple's was a step back by being non-removable. I absolutely loved having a removable battery, because I could bring 2 or 3 with me on a long trip and never have to charge the phone. Apple was less functional in that way.