http://en.wikipedia.org/wiki/S-75_Dvina#Countermeasures_and_counter-countermeasures
Sounds like they were firing blind. The SA-2 was a command guidance missile (about the least effective kind), the fire control radar would send steering commands to the missile. To counter it, all you need to do is jam the uplink, or the fire control radar. From the transcript it sounds like the Vietnamese were firing blind, without command guidance, hoping for a lucky hit, or to cause the B-52s to panic and either abandon their bomb runs or have crashed caused by evasive maneuvers. Sounds like the b-52s maintained discipline.
The NVA were indeed firing blind by this time. A combination of improved mission doctrine, effective jamming techniques and coordination of BUFF strikes with Wild Weasel strikes had made the SA-2s’ radars all but useless.
When the Linebacker campaign resumed after a brief cease-fire, the AF had learned a lot from the prior, not-quite-so-successful campaign. One thing they’d learned was to keep the BUFFs bunched together to maximize the effectiveness of their radar jamming. However, there was one maneuver (the post-bombing turn?) that was still vulnerable (I forget why...maybe something to do with the jamming coverage).
Also, the bomber force employed was a combination of B-52Ds (that had been deployed to the Vietnam theater for a long time, and had both the “Big Belly” bomb capacity increase and improved ECM equipment) and B-52Gs (which did not have the D-model upgrades, and comparatively inexperienced crews). IIRC the G models took the bulk of the hits in Operation Linebacker.