Many people apply the word 'myth' to anything they haven't experienced. Everything is impossible until it happens.
I have seen toxic gas pockets build up under various circumstances. It can happen when use have driftwood on slate with a sand substrate, with thick sand, or even with only 0.5-1" of sand. Fortunately I have caught all of mine before it got so bad that it killed everything, but I have had customers who were not so fortunate. I have had it get so bad that when I found it, it made the whole tank (and room, and that part of the house) smell like skunk (hydrogen sulfide). It is definitely worth doing something as simple adding some trumpet snails to prevent.
The hydrogen sulfide is produced when there isn't enough oxygen, but that doesn't mean that being exposed to the water column will neutralize it instantly.
IME Malaysian trumpets are easy to keep under control. Worst case scenario they do become a plague of locusts in your tank and you buy some loaches.