It's still cannon for regular tadpoles. But as they explain in this game, these are not regular tadpoles. They have been altered by Netherese shadow magic.
For what purpose? We don't know yet. All we know is they are not acting like normal Illithid tadpoles. Instead of consuming their host's brain and starting the ceremophosis process, they are just hanging out, communicating with their hosts through dreams, and granting special powers.
It may be as simple as an attempt to take over host minds without altering their bodies through ceremorphosis, so they can infiltrate the various societies of Faerûn in a way they can't in their mindflayer form. Like pod people in a '50s sci-fi movie. Get them into positions of power and you've got a fifth column to help in the invasion to come.
Or it could be something else. We really don't know if the Illithids went to the Netherese for help with this, or if this is a Netherese plot and they are just using the Illithids for their own purposes.