Of course, he is much more famous as a fiction writer than as a mathematician and it can be easily argued that much more important mathematicians (e.g. Grassmann & Clifford and many others) are missing from that timeline, but he certainly cannot be considered as a non-mathematician.
(Also, Yau is there.)
They added Katherine Johnson who developed no new theories or applications on Math or Computer Science, but left Margaret Hamilton out (and she was the pioneer and creator of an entire new field!)
Always bothers me the same happens in Civilization games with scientist lists.
I guess I just have an inclusionist mindset with these matters. Popularizers, esotericists, eccentrics, early pioneers of new thought-tech, and people in closely associated fields should at least be considered in projects like this. Contributing to mathematics culture seems important to me, not just contributing to mathematics achievements.
Citation needed. You might be able to argue that it had its beginning in EE departments, but CS as a field did not spring from math at all. Seriously there is a difference between formalizing something in mathematical language - which happens to almost every technical field - and having that formalization be considered an advancement in math itself. This is exactly my point about throwing everyone "technical" into the mix. After all, the formalization of chemistry requires lots of math, so Marie Curie must be on the list of mathematicians, too!
By whom? Obviously the poster did. Lots of people think of him in connection with his work in logic, which isn't research-level, but I think of him in the context of, e.g., https://en.wikipedia.org/wiki/Dodgson_condensation , which to my somewhat amateurish knowledge of the subject was a research-level contribution for its time.
So what? The site is a Timeline of Mathematics, not a Timeline of Mathematical Research. Surely notable new applications of mathematics belong on the timeline, regardless of whether they involved new research or not.