"Not a member" means that he was not a member of the Society of Friends at the time of his death. It may be that he had previously been a member, but had been 'disowned', perhaps for marrying a non-Friend. Or he may have been attracted to Quakers and attending meetings, but without actually applying for membership. Or it may be that a close family member had been a Quaker.
I can see that the Quaker registers in RG 6 have 10 records relating to Guthrie burials (of 7 individuals), but no births or marriages, so it's unlikely that your Guthries were ever in membership. These individuals look like they relate to a single couple and some of their children. My guess is that one or both of the parents had been attracted to Quakerism and had been attending Quaker meetings, but had not applied for, or been accepted into, membership.
Your guess about the Pritchard name is a possibility, in that it could be a name on the mother's side of the family. I can see that there were quite a few Quaker Pritchards, including some also buried at Bunhill Fields. But there is no birth for an Eleanor Pritchard, who could qualify as the mother in your Guthrie family. It could be a maternal-line name from an earlier generation, but equally it might not necessarily imply anything about Quakers in the family.