Most supertankers are not capable of traveling through the Suez and if hostile forces were to seize the Suez, they could not hold if for more than a week as many nations would enter into a military pact to open the canal.
Yes, it's true some supertankers can't make it through the Suez, but I wouldn't say most can't make it. Like the Panama Canal's limitations, which are known as Panamax, the Suez has "Suezmax". The Panamax restrictions are even less forgiving than the Suezmax restrictions, which are depth (draft) restrictions more than anything else. I think it can accommodate a draft approaching 70ft - that's quite a bit, much more than even a US supercarrier needs - I know, I've been through it four times on a carrier.
In any event, the Suez still sees a tremendous amount of oil shipping each each, and it's recognized as a "chokepoint" in the oil supply chain.
I also agree that it would be virtually impossible for Egypt to "hold" the Suez against an allied defense, but reactionary forces could play havoc on the shipping channel guerrilla attacks. There are plenty of places where you're exposed to shore-launched RPGs, to say nothing of mortar or less-mobile artillery & rocket attacks.
All but the largest tankers can pass through the Suez now that it has been deepened 66ft.