As I sad, I would expect the barriers to be needed unconditionally on a core migration. The fact that there is a special flag that is set when (and only when) the cache control instructions are used seem to point to some special handling specifically for those instructions.

Edit: having read the page for the nth time, I think I finally understand your point. The code using the cache instructions had an explicit barrier already, but it would be executed on the wrong thread.

I know nothing about the arm memory model, but likely the dsb sy barrier is a stronger barrier than needed for intercore communication, and it is needed for IO serialisation, for example with an mapped PCI device.

So yes, the article is clear and likely correct, I just failed to understand it fully originally.