Every company he starts/runs is to enable viable Mars civilization. SpaceX = get us there. Tesla = cash cow for SpaceX + Mars ground transportation. SolarCity = energy infra for a Mars colony. Boring = radiation protection for Mars habitats. Neuralink = moonshot cash cow, advancement of neural medicine for Mars colony.
Trying to advance/safeguard humanity is the most funsies thing one can do.
If you definition of "X is for Mars" is so broad as to include "brain-computer interfaces just like on Earth, but done on Mars" then yeah I suppose pretty much everything is done in the service of Mars. For example, Google is working on having the best search engine available to use for search on Mars. Facebook wants to build the best social network, for Mars. Etc.
The reason an advanced Neuralink could be invaluable on Mars is because all the complex machinery required for diagnosis of brain-related ailments will be prohibitively difficult to set up on a Mars colony for a good while. The brain is the least understood ogran in our body - Elon is trying to accelerate development and generate awareness in the field by giving it an honest go with his own company. A BMI like Neuralink would also be a multiplier of human effort when there is a limit to how many colonists we can ferry per unit time.
Neuralink aside, you seem to not have mentioned the other companies - I wonder what you think of my theory regarding them?
Trying to advance/safeguard humanity is the most funsies thing one can do.