You can get close, though not clean. The number you want is market-wide occupancy, not just listing count. If active listings rose 40% and market occupancy fell 13 points while total booked nights across the market held flat or grew, that's supply dilution and every host is splitting the same pie into more slices. If booked nights across the market actually fell, demand shrank and adding more listings just made it visible faster. Paid market data tools publish booked-nights series at the zip or submarket level, and that series is the one to watch rather than the RevPAR headline.
Both of those tools infer occupancy from calendar blocks, and a blocked calendar can mean an owner stay, a maintenance hold, or a direct booking. That inference error gets worse in markets with lots of second-home owners who list part-time, which is exactly what a beach market looks like. So treat the level as soft and the direction as usable.
The part that matters more than the diagnosis: rate discipline against a diluting supply base is a slow bleed, because the new listings usually undercut on their first season to buy reviews and then hold. Your $210 is being compared to their $170 by the same search ranking. Holding rate protects the ADR line and costs you the booking.
Also check whether anything changed in the local permit posture during those two years. A permit cap that grandfathers existing operators changes the supply forecast completely, and a cap that's about to be imposed usually triggers a rush of new registrations right before it lands. Pull the town's registry count by year if they publish one, and confirm what the ordinance actually says with someone licensed in that state before you price a whole season around it.