Before answering specifics, let me state that in giving these ratings, I am assuming that the scores have the actual distribution that the dice say they do. Therefore your chance of having an 18 in ability score is 1 in 216. A large high school with 2000 students will have 10 students with CON 18, 10 with INT 18, and so on.
Given the billions of humans that have lived on earth, some remarkable people are going to have multiple 18s - 1 in 40,000 will have two 18s, 1 in 8,000,000 will have three 18s, 1 in 1,600,000,000 will have 4 18s. There are probably 5 people with 4 18s alive today, by that measure!
With this in mind, I don't really think Alexander's 18 Constitution is arguable. He was tireless. Inexhaustible energy. Survived incredible wounds. It's worth reading Robin Lane Fox's description of the wounds Alexander suffered and survived - prior to marching through the Gedrosian Desert. His death by disease was painful and lingering because he took so long to die.
As far as his Dexterity, it depends to what extent you believe he was genuinely an Olympic-caliber runner/athlete. I think his personal heroics and survival on the battlefield certainly justify it. Calming Bucephalus down was generally described as an act of Intelligence (he figured out what made the horse upset), not Charisma; in any event, actually riding a horse requires a lot of Dexterity (and Strength and Constitution).
On a broader note, I was thinking of doing a series of these as blog posts to promote Domains at War: Julius Caesar, Hannibal, Genghis Khan, etc. Interesting idea or lame?