but addDicType works fine
for example

forest=forBiomes("minecraft:birch_forest","minecraft:birch_forest_hills","minecraft:mutated_birch_forest","minecraft:mutated_birch_forest_hills")
forest.removeDicType("FOREST")
forest.addDicType("MAGICAL")

results in Birch_Forest (minecraft_birch_forest).json being

"Dictionary Types": [
    "FOREST",
    "MAGICAL"
  ]

broken with both
BiomeTweaker 1.12.1-3.1.306 (latest curse version)
BiomeTweakerCore 1.12.2 1.0.23
and
BiomeTweaker 1.12.2-3.1.311 (latest jenkin's version)
BiomeTweakerCore 1.12.2 1.0.23

Turns out this is appearing in the console:
[SuperScript]: Failed to find meaning in command removeDicType("FOREST"). It will be ignored.

must be. Does this page exist in your time? https://github.com/superckl/BiomeTweaker/wiki/Basic-Script-Commands

I swear it wasn't there before... Also, Bearinstein Bears. Now to clean up the stupid system-assigned biome descriptors on my hell biome.

I tried to find "removeDicType" in the documentation, and while it would be really handy to have, it does not appear to actually exist at this time. Perhaps you are a time traveler.

Turns out the issue was a change in the command name. The name you should be using is removeDicTypes. This has been changed on the wiki as well.