The message is logically wrong. "All heap blocks were freed -- no leaks are possible" should be: "All heap blocks were freed -- no leaks were detected"
I am not sure to understand what is really wrong in this message, but it is like that since at least 2009 and nobody complained up to now about a difficulty to understand it. As tests are depending on this output, and possibly some users have scripts checking for this string, it looks better to not improve this message, as the cost/benefit seems not that good
The message as it stands is really what I intended. If all heap blocks are freed then it isn't possible to have any leaks, since there are no blocks to leak. Hence the wording.