I can't find a specific SQF code that requires an airlock or second door between an exterior entrance to a processing area. Closest I can find is 11.4:
11.4 Personnel Processing Practices
11.4.1 Staff Engaged in Food Handling and Processing Operations
11.4.1.1 All personnel engaged in any food handling, preparation, or processing operations shall ensure that products and materials are handled and stored in such a way as to prevent damage or product contamination. They shall comply with the following processing practices:
i. Personnel entry to processing areas shall be through the personnel access doors only;
ii. All doors are to be kept closed. Doors shall not be open for extended periods when access is required for waste removal or receiving of product/ingredient/packaging;
iii. Packaging, product, and ingredients shall be kept in appropriate containers as required and off the floor;
iv. Waste shall be contained in the bins identified for this purpose and removed from the processing area on a regular basis and not left to accumulate; and
v. All wash down and compressed air hoses shall be stored on hose racks after use and not left on the floor
A second door is definitely preferable, but I've consulted with businesses that did not have a second door after the entrance and they passed SQF audits. In their case, the door opened into a warehouse where a processing area was about 50 feet from the door. The building was a large storage warehouse with a small processing area partitioned off with curtains from floor to ceiling. They installed a handwash station and lockers for smocks directly inside the entrance, and that appeased the auditors in this case. Product was a low risk non-RTE product, which probably makes a difference, in addition to the way they segregated the processing area. It was designed to protect processing from the storage area, but was deemed effective enough in their case.