Does introgression require interspecific hybrids to be fertile? How is this possible?
Does this mean that the interspecific hybrid is fertile?


Yes


  But is it possible, that an interspecific hybrid is fertile?


Yes

You might be confused by the idea that by definition two individuals from different species should not be able to interbreed. If so, then you should have a look at How could humans have interbred with Neanderthals if we're a different species?.