What is meant by "neurotransmitters flooding the brain"?
It is very likely what they mean by "neurotransmitters flooding the brain" is the phenomenon called Volume Transmission.

I paraphrase from this source:


  When a neurotransmitter is released at a synapse, it reaches its
  highest concentration inside the narrow space of the synaptic cleft,
  but some of it is certain to diffuse away before being reabsorbed or
  broken down. If it diffuses away, it has the potential to activate
  receptors that are located either at other synapses or on the membrane
  away from any synapse. The extrasynaptic activity of a
  neurotransmitter is known as volume transmission. It is well
  established that such effects occur to some degree, but their
  functional importance has long been a matter of controversy.