it's default set to bang bang, since the #define PIDTEMPBED is commented out.
code snippet straight from stock ft5 r2 marlin:
//============================= PID > Bed Temperature Control ===============
// Select PID or bang-bang with PIDTEMPBED. If bang-bang, BED_LIMIT_SWITCHING will enable hysteresis
// Uncomment this to enable PID on the bed. It uses the same frequency PWM as the extruder.
// If your PID_dT is the default, and correct for your hardware/configuration, that means 7.689Hz,
// which is fine for driving a square wave into a resistive load and does not significantly impact you FET heating.
// This also works fine on a Fotek SSR-10DA Solid State Relay into a 250W heater.
// If your configuration is significantly different than this and you don't understand the issues involved, you probably
// shouldn't use bed PID until someone else verifies your hardware works.
// If this is enabled, find your own PID constants below.