It did not detect methane lakes.
It found that Titan was shiny (in radar terms): that is, the reflections were from a smooth surface rather than a rough one, and at the same time not very intense.
As a result (quoting the 2003 New Scientist article Radar reveals Titan's methane lakes linked in one of the comments to your question), “some researchers believe that these are methane lakes, sitting in impact craters.” Other researchers may think of different explanations.
If we come to know, from other evidence and reasoning or just by elimination, that there are methane lakes on Titan, then Arecibo can claim to have seen them. But by itself it is providing no evidence that they are methane, or even that they are lakes.
Equally, if someone had launched a space telescope in 1961 and produced high-quality colour images of Mars, they could have claimed, on the Arecibo principle, “First detection of the seasonal vegetation cycle on Mars”, since at that time most scientists believed that that was what the seasonal colour changes were.