If you haven't noticed, people come here to kill time. If you're killing time then you're not being productive, therefor the people who are heads down trying to launch their startup before they run out of runway are not going to be here until they're ready to market their product.
This is far from common in practice and it is only applied sporadically. Something like allocating formatted strings for the HUD is IME much more common (and done in UE5/C++ too, so not even a C# forcing GC excuse).
Have you considered that maybe Matt isn’t all that surprised by this optimization, but he is excited about how cool it is, and he wants readers of all backgrounds to also be excited about how cool it is, and is just feigning surprise so that he can share a sense of excitement with his audience?