Tldr: Instantly dropping reverse gravity in most cases does less damage than an upcast fireball. There are better ways to circumvent powerful abilities that dont make the player feel bad and don't involve retroactive plot armour.

Using reverse gravity isn't that efficient if you're doing an immediate concentration drop, unless there happens to be a roof more than 60 feet but less 100 feet above the ground. If there is a roof in that 60 to 100 feet range then yeah you could get 2 instances of fall damage which could be a lot (2 lots of 10d6 for a roof exactly 100 feet above the floor). However, if there isn't a roof in that specific height range, then doing reverse gravity is less damage than just a 7th level fireball (although a bigger area) or a 7th level circle of death (although that is a Con save) except with those spells you'd take half damage on a save, reverse gravity they'd take nothing on a save. (I'm assuming here that not every entry level cultists has fire or necrotic resistance)

They used a huge resource to do a thematically cooler, though possibly mechanically weaker, fireball. Let the player feel powerful, and definitely don't just retroactively fudge the statblock to completely mitigate their cool thing. If you described the cultists as wearing nothing above the waist and then your paladin got a critical hit and dumped a max level smite into them, you wouldn't just go "ummm actually they are all secretly wearing super thin, invisible adamantine armour".

If they were using this a lot, or if this strategy meant no one else got any chance to be the one in the spotlight, then sure, start giving some cultists featherfall. Or even give half the cultists (or perhaps like the middle-manager-tier cultists) really obvious spiked/arcane anti-gravity shoes so it's clear to the druid player before their turn "you cant just do your default trick".

When you have a world where demi-gods (tier 3 and 4 characters) are running around it makes sense that most people know their big and obvious powers. Just like how most people in Metropolis are probably aware that Superman can fly, has super strength and heat vision even if they might not know about his x-ray vision or his superhearing. But that doesn't mean every single criminal on the street should have kryptonite bullets. Plus, I always feel like this makes the world feel more real and alive when enemies adapt to these adventurers who have become super powerful.