It was uniformly proven to be the thinnest; others that were thinner to a point had a greater maximum depth (camera lens). Hence, this was not a mistake; in no way can the Pixel's advertiser be seen to be correct, as resolution and DPI are completely different characteristics.
Was it? I don't recall reading anything that stated it was. Do you have a source? Further - if it was the truth - why did Apple immediate (even at the end of the keynote) change their line to world's thinnest iPhone" - and put that on the website. Why not world's thinnest smartphone? It was sure stated a lot before the last few minutes of the product launch...