I'm not sure exactly what happened here but it looks like someone took some pictures of Ahvenisto Swimming Pool back in 2008 and said that it was the abandoned Olympic site. At least four different articles reference it, most likely all based on each other.
A comment in this one, says "This Lake Ahvenisto was the venue of Modern Pentathlon and athletics swam the swimming part in this pool. As you can see from picture is rather bad condition, and there has been discussion of repairing it, but who's gonna pay the renovation." So maybe I've been looking in the wrong place!
Back to Wikipedia, it says that the Mod Pent for 1952 was held in a Finnish city called Hameenlinna. Google maps puts Ahvenisto right outside of Hameenlinna so that could very well be right. A close zoom on the map here shows what certainly looks like the abandoned pool.
Instead of a grand sweeping conclusion about how Finland allowed their Olympic sites to decay, I think we can conclude that one piece of a satellite competition is abandoned and going back to nature. That blunts the impact, at the very least.