I personally don't understand why some people have questioned your claim that INTPs are "devoid of emotion," especially considering that they provided no reason for the case being otherwise, but that's a bit irrelevant to the topic at hand. Human memory, from what I understand, is much more complicated than the quote you had posted suggests, although that post does contain some valid points. Repetition and recitation are powerful tools for strengthening and maintaining long-term memories, and events that one perceives emotionally tend to be more memorable than events not perceived emotionally if all other factors remain equal.
However, factors such as concentration, context/setting, learning rate, the usage (or non-usage) of mnemonic techniques, meaningfulness, the degree to which one understands information related to what one is currently learning, intrinsic interest, extrinsic interest, and, yes, personality type--all of these and other factors can affect the strength of your memory regarding a topic or piece of information.
My guess as to why INTPs can be so poor at remembering names is because, as others have said, lack of interest. Why bother putting forth effort to remember something that you don't value? I would also imagine that INTPs in general tend to be more interested in focusing on their imaginations than they do on external reality, thus rendering their memory for some things considered important by Sensors--who focus on concrete minutia of external reality and on the present world--to be rather subpar. For example, it seems to me that Sensors are capable of providing driving directions in far more detail than I can, while their memory for matters that might interest an INTP oftentimes seems poor, if not non-existent. Interest and concentration are key.
As for the biological aspects of human memory… that I currently understand very little, but I don't suspect that our notorious forgetfulness of names is necessarily linked exclusively to our phlegmatic nature. For some highly social or emotional people memory of names may be regularly enhanced by the emotion involved with their meeting another person, but I suspect that the chief factor responsible for memory of names is interest. And while I won't contest that INTPs can be rather unemotional at times, the fact is that they do have emotions, and thus are likely to have their memory for names enhanced when their emotions regarding a person are aroused (e.g., meeting a significant other or discovering that someone scammed you).