The water damage to the Sphinx doesn't mean it's really old. It just means it has suffered water damage. The rock is a mix of hard and soft stone, something that get's glossed over in the reading or reporting. And effects of water on stone have been largely misreported or outright lied about in science for years. Stellagmites and Stellagtites of a length of 5 feet can be shown to grow over a period of less than 50 years, something that has been said takes thousands - even millions of years (for the sake of the evolution crowd no doubt). The weathering of the rock if for no other reason than what it is, could likely be counted for on a period of a few hundred years or a few thousand. There's no real reason to expect longer than that given that scientists can't even honestly account for any exacting level of rainfall or flooding beyond the last 100 years. They're making guesses and not even what I'd call "educated" ones.
Their guessing is based on the same faulty logic that discusses "anual rings" in polar ice core samples. Glacier Girl was covered in hundreds of feet of snow and ice pack in less than 50 years with hundreds of "annual rings". By faulty science, we'd have to call recent recorded history an utter lie and place WWII back several hundred years. That or admit the scientists are clueless and bs us to look like they know what they're talking about.