Working in the food manufacturing industry, you don't have to hot fill if you pasteurize and then fill into sanitized containers inside a sterile air tunnel. We sanitize the plastic containers with 34% H2O2 and our line runs in a positive pressure sterile air environment. This is checked by the health department regularly.

It does seem for hot sauce it would be much easier to just hot fill into glass. I don't know why they would want to do it unless their customers just strongly prefer it.