It's because it's not written in QBasic. It's written in a variant of QBasic called QB64 [1] - a spiritual successor to Microsoft's QBasic intended to run on modern 64-bit machines.
I went over the BAS source the last time this was posted a few weeks back and it has a lot of keywords that are specific to QB64 so unfortunately you can't run this on a true DOS machine (or DosBox).
I went over the BAS source the last time this was posted a few weeks back and it has a lot of keywords that are specific to QB64 so unfortunately you can't run this on a true DOS machine (or DosBox).
[1] https://qb64.com