Honey degrades in temperatures over 40°C. When I cook the porridge in the morning, it definitely has more that 40°C. If I mix in the porridge a tablespoon of honey just before eating it, I guess the porridge can have 70°C or so, will it survive such a short exposition to the higher than 40°C temperature without degradation?
There is no single well-defined idea of "degradation" of honey. I guess there will be some temperature above which it stops being honey, but that be a charring temperature somewhere above 200 celsius.
This doesn't mean that honey stays the same all the time. It is very complex, and some compounds can certainly get destroyed when heated. Some are even fragile enough to get destroyed without heating, just from staying around.
If some author has decided that some compound is especially important for some reason (taste, health, whatever) and has measured that it gets destroyed at 40 celsius, than it will get destroyed no matter if you heat it in porridge or not. If this compound is important to you for some reason, don't put honey in your porridge, it will be destroyed there too.
But if for you, hot porridge with flavor-degraded honey tastes better than cold porridge with full-flavor honey, or hot porridge without honey, then the flavor degradation of the porridge as a whole obviously happens when you don't add the honey to the hot porridge, so it doesn't make sense to stop doing it.