Pizza dough with a mixture of strong and plain flour turned out surprisingly well. Lucky? Better with lower gluten?

As you probably know, various flours contain varying amounts of protein. When water is added, and the proteins are hydrated, gliadin and glutenin combine to form gluten. Gluten provides the structure and allows for stretch and rise. That is why a rest period after mixing is important. You might find this informative. The folks at Serious Eats took a look at various flours and their impact on pizza crust. While flours with different percentages of protein all work, the end result is somewhat different, and mostly boils down to preference. I make a lot of pizza. I usually mix and let rest for 24 hours before portioning, waiting a couple of more hours, then stretching and preparing the pizza. A long rest and a light touch are important to maintain a workable dough. I mainly use Caputo type 00 flour, but have also used AP flour and combinations of flours. In my experience "stretchiness" has more to do with hydration, time for the dough to relax, and a light touch. Type of flour impacts flavor, rise, and texture.

You can certainly make pizza without strong flour. In fact, there are many countries in the world (including most of continental Europe) where nobody outside of the food industry and a few afficionados has heard of strong flour at all.

Bread with lower gluten flour is softer, with a "cottonlike" softness. And it can easily build a hard crust, which is also crispy at the proper temperature. The dough is easier to roll, because it's less elastic, and doesn't "jump back" as much while you shape it. Bread with higher gluten has a different crumb quality, being somewhat tougher, more translucent and less absorbent. It can also be used with higher hydration, and it can hold larger holes, which is important for e.g. ciabatta.

You can make great bread out of both, but it's different bread. And you can make pizza crusts out of both. People looking for the qualities of high-gluten bread (or crust) will be disappointed if they are served a low-gluten one. But if you happen to prefer the low gluten one, then just keep making it.

I've made pizza dough with just plain/all-purpose flour (100%) and no strong/bread at all, dozens of times. I've experienced no problems. I never made it using all strong/bread flour so can't compare using my recipe, but I see no reason, based solely on my experience, why you should need more gluten in the flour for this purpose. Since you have a lot of plain flour on hand, experiment and make just a small pizza (half batch) using 100% plain flour and see for yourself.