> how an interpreter would have to proceed to execute Recto code, it would still parse it into an internal representation and then execute that, and this internal representation no longer uses the 2D layout of the code.

Adding parallel execution will make it true 2D at execution time. Even though it's sequential code. More over it can be logically, architecturally 2D while being 1D from interpreter's standpoint. If it uses cooperative multitasking with 1 running task at a time. Or simply 1 core processor.