Your code is using the default SYSTEM_MODE(AUTOMATIC) which will by default hook you up to the Particle cloud with all the handshaking and Particle feature enumeration.
If you won’t need any of that, use SYSTEM_MODE(MANUAL), but you’ll still see more than just one UDP packet as you’ll still need WiFi.connect() which usually performs some negotiation with your WiFi AP including DHCP requesting an IP.
To further cut down on that, you could use WiFi.useStaticIP().
If you want to use the low-power features of the Broadcom module before Particle has their APIs for this ready, you’d need to dig into the Broadcom WICED SDK.