That previous thread was remarking on a specific bug affecting a specific switch model on a specific version of software which caused the fans to run at full tilt all the time.
In Cumulus Linux, the fan speeds dynamically adjust themselves based on external temperatures.
As Attilla states, there is no supported method for lowering fan speeds.
Individual platforms set cutoff thresholds for these sensors and critical temperatures for each sensor in the platform files.
You can view them here: cat /lib/python2.7/dist-packages/cumulus/platforms/mlnx_SN2xxx.py
But changing these values can void your warranty as you are effectively preventing the box from appropriately cooling itself so I would not recommend doing this, even for testing.