The instructions state:
include
Specifies a list of file names or patterns to be included in the program.
These file names are resolved based on the directory where the tsconfig.json file is located.
Source: https://www.typescriptlang.org/tsconfig#include
Therefore, according to the documentation, it should consist of an array of files and not both files and directories.
Although it might also function for directories due to some unmentioned method of using that value by the compiler (which I am uncertain about, as mentioned in the comments below your question), I would not recommend relying solely on the directory name just to save a few characters. Utilizing undocumented features always carries the risk that future updates could eliminate these features without warning, technically not constituting a breaking change.
In essence, while the shorter version may appear to work, if it deviates from official documentation, it's best not to depend on it for long-term use.