You're confusing two different decisions: 1) to relocate the capital to a new federal district in a rural-ish area, and 2) to decide where that federal district was going to be located. You're talking about #2; I'm talking about #1. Washington's recommendation to pick a location in the south came several years after congress had already decided #1. If Washington had not pushed for it, DC would have been built in a different previously-undeveloped area, possibly in the north. Again, see https://en.wikipedia.org/wiki/Pennsylvania_Mutiny_of_1783