Well, it does state that tectonics are a likely cause, so what's puzzling you? Obviously if it sank for that reason, then the sea level rose by the same amount as the land sank.
If it was drowned by a tsunami, that implies there was an earthquake, so that would account for the tectonic drop. The sea floor can rise or fall after a massive earthquake - such a thing happened off Sumatra during the 2004 earthquake and tsunami, where the sea-floor level dropped by a number of metres (I don't know the exact figure) and rose by 10 metres after the Japan earthquake in 2011.
I don't see any conflict in the article's conclusions.
I'm simply raising (no pun) the possibility these ruins could also be much older. It would also explain how man made structures end up 27 feet below the water. If for instance Göbekli Tepe was at the time build on the coast it would now be much deeper under water than 'a mere' 30 feet. However if this possibility is excluded really only tectonics remains as a plausible explanation how there are walls and pieces of stairway on the sea floor.
so what's puzzling you?
What dictates the estimation of "about 1,100 to 1,500 years old"?
Is it science or dogma?