It is true that both tools are mutable and that hypothetically their potential use cases will overlap entirely.
The reason this will not occur though is a consequence of one tool aiming to provide a general environment by design and the other tool aiming to provide a general environment for a specific use.
Less abstractly, consider the difference between having a factory which produces a specific item and having a factory which can produce any factory.
Possibly the specific item the factory produces is sufficient for the recipients of the specific item, and further that specific item may even make other items the recipient possessed moot.
However, the factory which can produce any factory can still (and possibly already has done so) produce the specific item.
Emacs' design (and Light Table's) is much more similar in breadth to the factory which produces other factories.
Sublime Text is more similar in scope to a factory which produces a single item.
The difference in design choice leads to Sublime Text and other text editors being comparable in features and it is also why Sublime Text can replace Emacs' text editing functions.
At the same time the design choice also leads to Emacs being a RSS client but Sublime Text not also having that functionality (or being reasonable to expect), despite the potential for the functionality to exist in both tools.
The difference between the two goals is why Light Table is much more intriguing to me than Sublime Text is despite the utility Sublime Text provides and how well executed Sublime Text is.