I also ran a dual Celeron system overclocked to 450mhz - it was great value in 1999. Abit even launched a motherboard that let you run dual Celerons without modifying the processors, the legendary BP6:
https://en.wikipedia.org/wiki/ABIT_BP6
This was first board to let you use unmodified Celerons, the "hack" to let dual CPUs work with those chips was performed at the motherboard level, no CPU pin modifications needed.
You are forgetting the massive price difference though. For sure a P3 was great if you had an unlimited budget, but a quick look at pricing sheets for September 1999 shows a 600mhz P3 at ~650 dollars.
The 300mhz celerons, easily over-clockable to 450/500mhz, where only ~150 dollars each. These prices are in 1999 dollars too, I haven't adjusted for inflation.
It was the value proposition, not the outright performance that made dual celeron builds attractive, especially in an age where we were having to upgrade far more often than we do today to keep up with latest trends.
In 1999 I vividly remember not being able to afford a P3 build, was largely why I ended up with the BP6. The P3 also had significant supply issues throughout its lifespan, which didn't help pricing at retail either.
The real problem with this setup was that a vanilla Pentium 3 would run circles around the dual Celerons. I had my Celerons clocked to something ridiculous at one point like 600MHz and still could not beat the Pentium.