London Density 14,670/sq mi (5,666/km2) (https://en.wikipedia.org/wiki/London)
Denver Density 4,519.94/sq mi (1,745.15/km2) (https://en.wikipedia.org/wiki/Denver)
London has a population density 3.25x that of Denver.
Why exaggerate when the facts are on your side? Or more specifically, why make an explicit numeric claim without confirming that it's correct?
I don't think it's particularly useful to attempt to pin down a fixed definition of the boundaries of either city, particularly one as amorphous as London. Suffice it to say that London is densely populated, Inner London is even more densely populated and rush hour on the tube is genuinely insane; Londoners regularly encounter densities that simply don't occur in most US cities outside of a football stadium and have a subsequent need for far higher throughput than 4G can deliver.
I personally dislike the phrase and try to avoid it. But if one is going to use it, I think it should be used correctly.
(I agree with jdietrich that the definition of "city" matters)
A fancy new 5G tower also has underutilized connectivity to it.
https://www.youtube.com/watch?v=_CTUs_2hq6Y
But I will add your data point to my opinion of 5G.
I have a gigabit pipe that works just fine on a speed test, but most real-world usage doesn’t go that fast because of bottlenecks elsewhere.
What happened instead is that it enabled Youtube, Skype, Facetime, and that did change the world.
Claims from "what I've heard" /are/ informed claims; your anecdotal information doesn't automatically beat other people's anecdotal information.