I have heard and understand from veteran officials in my area, that on a misplayed throw and the ball is within a "Step and a reach" of the fielder, the "continuning action" mantra prevails. If the ball is outside this area, call the obstruction and award the runner home. Generally, this is between 24 and 36 inches (2 to 3 feet) from the fielder.
In your sitch, it sounds like the ball was within reach of the catcher for the play to be made. There was nothing stopping the runner from sitting up and reaching over to touch the plate.
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