It's like being a paper tape Turing machine. Technically, you can do anything in the world. In reality, you're limited by how efficiently you, as an individual, can run back and forth writing zeros and ones.
Being a single founder isn't like building up a muscle that'll make you bigger, faster, stronger—there are world-level parallelism problems you can't accomplish alone (work and fundraise, work and promote, sleep and work).
Perhaps a better title is "single bootstrap'r." All you really need is to build something other people find interesting to get attention to attract validation to pull in other people who share your vision (who, since they are starting right after you, become defacto founders).
The single founder problem is more "Will you accept someone who is capable but hasn't found any smart friends yet?" and less "I can build a $50 million company all by myself. Just give me $100k. Please?"