It's really poorly phrased in the article but I assume what they meant is that LHR is in a time zone that sometimes observes GMT, and otherwise observes BST (British Summer Time) in the summer which is 8 hours ahead.
It's not exactly correct that SFO is 7 hours behind GMT because this isn't true for around half the year. It also isn't exactly true that LHR is in GMT.
Most of the year, SFO is actually 8 hours behind LHR. A few weeks are off due to different DST dates.