What they are saying is the stackalloc approach causes no GC pressure. You can run that in a tight / infinite loop with essentially no downside. Using a regular heap allocated array in the same situation will hammer the GC.

C# doesn't do escape analysis and automatically put things on the stack like (for example) Go does. However the stack size is limited in C# so I wouldn't suggest going too crazy with stackalloc and deep stacks / recursion. You don't want a stack overflow!