a decade ago you might be right, but a 245 marathon these days wouldnt get you very far, especially if that is your best discipline. KB has run 230 in an ironman and every single guy with a sliver of chance has easily run sub 240 off the bike in the past.
as others have mentioned an elite marathoner body would not be ideal for triathlon. the average height for a male in Africa is 5'6" and the average BMI is probably 17-18. simply too short and slender to be a great swimmer. triathlons also tend to be flat so having an exceptionally low body weight would be a huge disadvantage over a 112 mile bike ride on a course with almost zero climbs.