It's what a witness should say if he doesn't recall.

If he does recall and says "I don't recall," then he's committed perjury. The plausibility of Swarbrick's lack of memory varies. For instance, I find it plausible that he didn't remember the specific number of times he had talked to a person or the specific year that something happened. But the idea that he couldn't even approximate doesn't hold water. As noted by others, this deposition is a perfect illustration of how not to handle a slippery witness. The attorney could have kept hammering and either obtained some useful information or made Jack look (more) ridiculous. An attorney who gets that answer to a question the witness should be able to answer shouldn't just shrug his shoulders and move on.