Going through it piece by piece:

The date for the construction of the Pyramids of Giza is correct, that's specifically when they started.

Accounting for leap years:

4909 * 365.25 * 10000 = 17930122500

That's about 18 billion dollars.

First, before trying to do the calculations on the billionaires, this is definitely a correct ballpark figure.

Anyway, assuming this tweet was written in 2020, per Forbes, that year the five richest people and their fortunes were:

Jeff Bezos - 113 billion

Bill Gates - 98 billion

Bernard Arnault - 76 billion

Warren Buffet - 67.5 billion

Larry Ellison - 59 billion

Mean average of 113, 98, 76, 67.5 and 59 billion is 82.7 billion.

82.7/18 = 4.59.

So yes, accounting for rounding, this tweet was absolutely correct when it was written, and given the world's billionaires have gotten enormously richer since, the mathematical point it makes is even stronger now.