Can one describe the following as using the Past Tense?
Yes. There is use of the past tense in the extract:
I shrugged. It was clear that we 'd stumbled into a prehistoric gathering. The voice of a "drug expert" named Bloomquist crackled out of the nearby speakers:
This is all in the past tense. It is followed what Bloomquist said and that is given in direct speech. When he does this he uses the present tense as a generalisation
"... about these flashbacks, the patient never knows; he thinks it**'s** all over and he gets himself straightened out (present causative) for six months . . . and then, darn it, the whole trip comes back on him."
As far as the analysis of the passage is concerned, the contents of what he said are not important because crackled out of the nearby speakers: "... about these flashbacks, the patient never knows; he thinks it's all over and he gets himself straightened out for six months . . . and then, darn it, the whole trip comes back on him." could be replaced by "and he said something".