You haven't heard of Admiral Gilchrist? He was a Navy test pilot who flew everything from F6Fs to F-14s. I read his biography
Also since most military pilots aren't qualified to fly more than one aircraft type
Read that part again. *most* would be the part that doesn't make that statement all inclusive. Obviously, test pilots are an exception.

The big thing back in the 60s or 70s to prove squadron readiness was intercepting every single Bear that the Soviets would fly over.
It's not so much about proving squadron readiness as it is ensuring the Soviets didn't invade our airspace. Or in the case of an aircraft carrier, ensuring the safety of the carrier and it's battle group. There are major inspections done on all active duty squadrons to evaluate squadron readiness.
Gilchrist mentioned something about the starter cartridges. I think he said that while they could be used in emergencies, they also posed a high risk for fires. I think one of his F-4s burned up on the ground because of a starter cartridge.
I've seen lots of Phantom engines started with starter cartridges. Only one of those Phantoms caught fire and it was due to a cracked cartridge retainer. When the starter cartridge fired it blew the retainer to pieces rupturing a fuel line in the process. The fuel ignited and since the pilots didn't shut down either the battery or the fuel pumps before jumping from the cockpits and running like scared little girls, that plane burned much worse than it would have otherwise.
The F-4D Phantoms that sat on alert at SJ AFB not only had air hoses hooked up to the engines to start them in case of a scramble order, but they also had the starter cartridges installed just in case the external power unit failed in some way.
I'm not saying you didn't read what you posted, but I am going to call horse hockey on the guy that originally said it.