as a general rule changing the original game files will work, but there are always things that appear in the original game xml-files which are actually hard-coded (over-ridden) within the game's executing (DLL) file which we currently have no access to, or in an LUA file somewhere which can sometimes be brutal to track down and alter to allow the actual settings within the xml-files to apply. And with Firaxis there always seem to be a few things which are contained with the base game's xml-files but which are never used anywhere for anything.
as a second general rule mod-makers don't recommend altering the game's base files if we can avoid it because you never know when a game-patch is going to be issued that proceeds to wipe clean your changes in the original game-file you made.
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One hint though is to always look in file C:\Users\[YourComputerUserNameHere]\Documents\My Games\Sid Meier's Civilization VI\Logs/Database.log for error messages that might give a clue as to whether your changes are causing a syntax or other error when the game attempts to load the game-files.
This is the same file mod-makers will look in 1st when the game is not implementing our changes from within a mod.
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Another thought, though, is that if you actually used "FALSE" (ie, all in caps) in your alteration, the game might be looking for the standard presentation of booleans as either true or false, and is interpretting FALSE as if it is text and not a boolean.
It has been a while since I tried stating a boolean all in caps in civ5, so I don't remember how Firaxis xml-to-sql parsers tend to deal with this.