The area where The Falcon was. Flying over when Finn made his statement was open, and empty , desert; no wreckage, no hills, no boulders, just sand for kilometers around. He was not talking about Jakku’s unique terrain when he told Rey to fly low. It took them time to get towards the wreckage, they weren’t anywhere near it when Finn told her to fly low to the ground. For the record, flying low to the ground and to water it a tried and true military tactic in order to evade RADAR, no matter what the specific terrain. That too is how I know that Finn was not talking about the unique terrain of Jakku, but a specific flaw in the sensors of the Firdt Order TIE fighters that he knew about as a result of his first hand knowledge of First Order equipment.
Not only that, but do you really think that Finn would have the audacity to advise a native of Jakku, about the terrain of her own planet?
It was Rey, not Finn who decided to use the specific features of Jakku to their advantage, by actually flying into the wreckage of a Star Destroyer. Finn just told her to stay low to the ground, nothing more.
Edited by Tramp Graphics