The full context was:
Farm your team element’s Disaster to build your arsenal of Assault weapons
You can start 3 battles a day for each difficulty level, for a total of 6. Each battle can drop two platinum chests: one for spawning the boss, and one for MVP. SR weapons can drop from the platinum chests, and one of the three has Assault skill. There’s no difference in drop rate between difficulties, so prioritize Normal difficulty.
That was a comment only on SR weapon drop rates, not on the drop rates of any other items.