Why doesn't the instruction reorder issue occur on a single CPU core?

Question

From this post:

Two threads being timesliced on a single CPU core won't run into a reordering problem. A single core always knows about its own reordering and will properly resolve all its own memory accesses. Multiple cores however operate independently in this regard and thus won't really know about each other's reordering.

Why can't the instruction reorder issue occur on a single CPU core? This article doesn't explain it.

EXAMPLE:
The following pictures are picked from Memory Reordering Caught in the Act:

Below is recorded:

I think the recorded instructions can also cause issue on a single CPU, because both r1 and r2 aren't 1.

Answer 1

A single core always knows about its own reordering and will properly resolve all its own memory accesses.

A single CPU core does reorder, but it knows it's own reordering, and can do clever tricks to pretend it's not. Thus, things go faster, without weird side effects.

Multiple cores however operate independently in this regard and thus won't really know about each other's reordering.

When a CPU reorders, the other CPUs can't compensate for this. Imagine if CPU #1 is waiting for a write to variableA, then it reads from variableB. If CPU#2 wrotes to variableB, then variableA like the code says, no problems occur. If CPU#2 reorders to write to variableA first, then CPU#1 doesn't know and tries to read from variableB before it has a value. This can cause crashes or any "random" behavior. (Intel chips have more magic that makes this not happen)

Two threads being timesliced on a single CPU core won't run into a reordering problem.

If both threads are on the same CPU, then it doesn't matter which order the writes happen in, because if they're reordered, then they're both in progress, and the CPU won't really switch until both are written, in which case they're safe to read from the other thread.

Example

For the code to have a problem on a single core, it would have to rearrange the two instructions from process 1 and be interrupted by process 2 and execute that between the two instructions. But if interrupted between them, it knows it has to abort both of them since it knows about it's own reordering, and knows it's in a dangerous state. So it will either do them in order, or do both before switching to process 2, or do neither before switching to process 2. All of which avoid the reordering problem.