Hey!
I did hear this the other day myself.
As a staff i have never heard of this, so i decided to test it myself.
From my testing i found that having multiple accounts in a spawner chunk do not affect the spawn rate.
I also heard that if you have 1 player in the chunk, the spawner will always spawn 1 per cycle. I also found this false in my testing.
In vanilla more players in a chunk increase the mobcap, but the spawners on mana is not set by the vanilla configs. Example: The mobs are passive
Im not saying this is the truth, but its what i found from my testing. If anyone actually have any evidence or numbers to support their claims of this, i am very interested in taking a look at that!