Verizon’s 5G network isn’t good enough to cover an entire NFL stadium

Doesn't surprise me one bit. 5G won't be popular for years, and even then it'll only ever be limited to very high density areas ( think main streets of large cities, areas where super bowls may take place ).

Right now if you're in an area of relatively high pop density you can expect 4G LTE and decent speeds. In the future a new class of high speed will exist that only those who venture into highly populated areas will be able to enjoy. Stray off that path and you're on the lesser LTE version - a lot like going into the country nowadays ( unless you're on verizon )

Most stadiums implement cellular via DAS antennas and equipment local to the location. If the venue/site hasn’t planned for full capacity then this is no surprise. On another note stadium RF engineering is really complex and a fascinating discipline.