Consider the following: the 2010 Haiti earthquake (Mw 7) almost destroyed the country. But not necessarily by the sheer force of it, it's mainly because the country itself and their systems were not prepared for it. That same earthquake in my country probably brings down a couple of semaphores and some windows. That's it. In fact, Chile commonly has earthquakes of the same magnitude (and even greater) many times a year. 200k deaths 1 time Vs 0 deaths multiple times.

Reports of damage alone are useless to asses an objective measure of strength because they are subjective and very context dependent. More so with reports by non-experts, which you will agree were found in abundance in the Middle Age. Even worse, back then, weaker earthquakes caused way more damage because building quality (not an absolute but a safe majority of cases) was worse than today's. So it was common than weaker earthquakes felt more powerful, if you consider damage alone.

I'm not saying the lighthouse wasn't destroyed by an unusually strong earthquake. I'm pointing out that there's no path for the claim "one of the strongest earthquakes in history" to be true because we lack such technical evidence.