There is no "the" issue in airline accidents. There are always multiple factors, and all of them had to happen in order for the accident to occur.

Understaffing is absolutely a factor. Had tower and ground not been combined, the erroneous clearance probably wouldn't have been issued.

The ARFF truck not complying with the stop instruction is absolutely a factor. Had they heard and complied, the accident wouldn't have happened.

And there are likely additional factors that will come out in the investigation.

I recommend reading some final aviation accident reports from the NTSB to learn more about how these investigations proceed and what kinds of conclusions and recommendations they include.