To do that right they have to remove the thermostat.

They wait until the thermostat is open then you can see the old coolant being replaced

20+ years ago, when I was young and in school, I worked at a quick lube place (not Jiffy, but in that same vein).

The “techs” had minimal to no training. Made hardly more than minimum wage and received commission on extras sold. In my opinion they weren’t qualified to check the air in the tires let alone the lifeblood of the engine. Many dirty tricks were pulled, dirty oil smeared into air filter to coax a sale, yep. Wrong weight bulk oil in your engine, you bet. I’ve seen more than 1 car started after a drain and before a fill. I would trust these guys to do a good job changing the oil in a lawnmower. They also just did not care, whatsoever. There was no pride in work there.

Don’t do it, it’s not worth the potential for something to go wrong that you might not even see or notice. I get not everyone can do the work themselves, but choose trust in a mechanic at a reputable shop over the lowest bidder.

the sad part is that there is probably enough room to fit your fingers there with that stellar GM build quality in the C4 era

As soon as you start trying to add new coolant, the thermostat closes. making it impossible to continue the flush.