You are correct that you can "divert a flight" but not "deviate a flight."
The two words have different meanings. "Diverted" is correct here. "Diverted" means "an intentional, major change" such as going to a destination different from what was planned. If they expected to land in Gatwick and instead they landed in Luton, they were "diverted."
"Deviated" refers to a path or a route. It means that something took a different direction, perhaps only a slightly different direction, than planned. It also suggests a mistake rather than a planned change.
"When the captain took the noon sight, he found that the ship had deviated quite a bit from its course, and he ordered the helmsman to steer two points north to correct it."
"If you use a magnetic compass, you must correct for the deviation from true north."