From the full PDF:

– “We are concerned by the finding of a sub-lineage of over 350 sequences bearing seven spike mutations across the RBD […] in England. The finding of a long branch to this cluster of cases suggests viral evolution may have occurred during a chronic infection.”

Chronic means lasting a long time. The significance of the long branch is its lack of ancestry; none of the intermediate steps have been found in the wild, so this long branch developed, change by change, in an environment isolated from the wild, such as a single person.

Of course another instance of an environment isolated from the wild is a biolab, but papers that suggest lab escapes are, I’m told, unlikely to get past peer review; “too political”. Not that lab escapes are particularly uncommon or anything; remember that the first SARS got out four times; twice from the same lab…