Their tl;dr isn't quite right. Any physical computing system has finite capacity, so you're right, no physical computing system is actually Turing complete. Ho

Their tl;dr isn't quite right. Any physical computing system has finite capacity, so you're right, no physical computing system is actually Turing complete. However, in most languages (JavaScript, Python, Java, ...) there's no concept of an object's location in memory as expressed by a finite number of bits. In those languages, you can express programs which will create an ever increasing number of objects. Trying to run that program will eventually cause a crash as you run out of space, but the source code itself encodes a program which would allocate objects with no upper bound. That's not the case for C. In C, every object has a unique address. That address is encoded in a fixed number of bits. As such, you can't even write source code which allocates objects with no upper bound; the upper bound will always be 2 to the power of the number of bits in a pointer.