Harker says, "That's Renfield. What's he doing at the abbey?" He doesn't say "What's he doing here."

Therefore, Harker and Van Helsing-- who see Renfield from 300 feet away, with nothing between them but open land -- are somewhere else other than the abbey. This is not all that difficult.

In the book, as in its real-life inspiration Whitby Abbey, there is a cemetery adjunct to the abbey. The abbey is the structure, not the entire acre it sits upon. Guys!