Vendors marketing is getting overexcited with hyperbole and suddenly basic filtering such as access-lists are Deep Packet Inspection.
Packet munging for layer 2-4 is shallow packet inspection. Specifically, its when you match Ethernet MAC, IP Addresses and TCP/UDP port numbers but nothing else. At time of writing, shallow packet inspection is simple, cheap and part of the forwarding ASIC.
Devices that perform inspection at Layer 5-7 of the OSI model is deep packet inspection (DPI). This requires exponentially more complex handling of the data in the ASICs, awareness of data formats and flow operations. In addition, the applications that configure are complex and sophisticated (the CLI just doesn’t cut it).