The insinuation here is the obsessive behavior. The text calls it a madness so I'm guessing it was a sudden occurrence.
I'm not an expert on Autism or other mental situations like ADHD mind you but it seems pretty likely Thrasyllus became hyper fixated on boats one day and he basically became a harbor manager for it much like you see other hobbyists become conductors or train line engineers for toy trains and that before the very modern diagnosis of Autism came about these people were considered strange until the behavior could in some part be explained by neurodivergent behaviors/tendencies.
People often accuse me of being Autistic and it infuriates me to no end. I don't care if I am or not I don't give a shit about knowing. I'm perfectly fine without the label and I'll keep being fine.
I hate that so often we are encouraged to label ourselves in society and that just as often people seek these labels to fit in, gain community or otherwise make themselves more interesting by incorporating some quirky or less than common fact about themselves as a whole personality trait.
I am finding more and more that neruodivergent labels are becoming the new version of houses in basic bitch Astrology.