Celerons of that era were very easy to officially overclock generally because a great many of them were underclocked by Intel.
They ended up with far more of the more expensive 400+MHz capable parts than they expected to produce (presumably they were overly cautious when predicting how well the production process would work and therefor how many of the better rates chips thye'd get) but too few of the cheaper 300Mhz and 333MHz rated parts that were actually selling well - so they rebadged faster units as slower ones to fufil sales promises for the slower units.
This meant buying a machine based around a Celeron 300 or 333 (often referred to as "Silly-rons" at the time) was a lottery: you might have got one that is genuaninely expected not to be stable if run much faster, or you might have got one that Intel could have sold as a 450+MHz device.
As a side effect, those chips also helped launch Internet publishing - sites like the improbable Toms's Hardware, founded by a German M.D. if memory serves, cut their teeth on overclocking advice and benchmarks.
It wasn't so much as they were underclocked, but that Intel's process was so refined that they were often produced over-spec.
The other thing that made them so easy to over-clock was that, unlike the Pentium Pro, II and III, there was no on-chip L2 cache that would malfunction at higher speeds. All you were over-clocking was the CPU.
I had a dual-Celeron system that ran like a champ for the better part of ten years.
The binning process where higher end CPUs are marketed as lower end parts has been going on forever.
Only the first generation Celerons (which everyone hated) had no L2 cache. The second generation had 128KB of full-speed on-chip cache, which is actually why it overclocked well. The Pentium II and early Pentium III had 512KB of half-speed off-chip cache which struggled to keep up with the CPU when overclocking.
In fact, those were the last days of my "dinking" with hardware. The Pentium and beyond offered such diminishing rewards that it wasn't worth it.