The statement is believable to me once there's context added.
IE was a terrible browser. Everyone knew it and Firefox was better in every imaginable way especially for "web masters". The Firebug system was a godsend.
Firefox unseated IE because of how much better it was. Microsoft could not get its head straight in this particular case. Always a non-conforming, slow browser with far fewer features.
That's what it takes to unseat the default - you have to be multiples better before people take notice in serious ways.
And, Chrome did that beautifully by leaning into the one thing the masses cared about. Speed.
Those ads were so good - extremely viral. Did they show anything objective or real? Maybe... maybe not. It's what caught people's attention and then when they tested it themselves, they believed it.
Personally I tested it and it was, by a very noticeable amount - I was hooked. Took me a bit of time to get over the loss of Firebug, but mostly sentiment because Chrome's debugger was quite good.
If Firefox was the semi-truck, Chrome was the Ferrari. Both got me to A => B. With firefox I could carry anything, plugins, extensions, whatever I needed. But it was a lumbering giant. Chrome came with none of that at launch but it went fast - and people talked about it.
Those were the days.