I’m not sure where you get this model reference from. The official reference uses the code I mentioned above.
https://fluxml.ai/Flux.jl/stable/models/layers/#Flux.LSTM
https://fluxml.ai/Flux.jl/stable/models/layers/#Flux.GRU
Even an outdated version of the docs should not have some activation function for LSTM and GRU due to the way these networks work. There is no activation function in the same sense as there is for RNN or Dense layers.
In my opinion, this resource, which I found to be quite useful, makes it clearer why a user-defined activation function for LSTM does not make sense: Understanding LSTM Networks -- colah's blog. I hope this helps!