Well I have never heard of not riding them for a day or two after deworming but I can see why it would be relevant to horses who are not dewormed regularly - kill a bunch of bots or strongyles (each have their own effect on certain systems) and then get the horse's heartrate up and sure, those dead worms in the blood stream could stroke or clot and cause all kinds of issues. I've seen this happen with goats and dogs.
I am also conscious of my horses and what they may 'feel' as sappy as it sounds and don't ride them for a day or two after deworming, vaccs or teeth floating in order to give their bodies and systems time to recoup from the chemicals/sedatives/invasive procedures, etc.
If you deworm every two months, your horse's worm load (if he even has one) will be minimal and you could probably ride the following day. If you actually see worms in the poo and this is what prompts you to deworm, well I'd be laying off any form of exercise and deworming with a more aggressive regime until a fecal count shows clear. Other than that, you could get in the saddle the next day with no problem.