Hi
Yes, you are looking at the marriage in the parish register. They could either be by banns (called on three Sundays preceding the marriage), or by licence. A lot of those Withyham marriages seem to be by licence.
With licences, what we see today (if we are lucky, and they survive) are the bond and/or the allegation.
Finding a record of banns being called or a licence being issued doesn't in itself mean a marriage took place, you are right. But William and Sarah certainly married.
Here's an article on marriage licences by the great Anthony Camp, former Director of the Society of Genealogists -
https://familysearch.org/wiki/en/Marriage_Allegations,_Bonds_and_Licences_in_England_and_WalesEDIT - Another possibility is that Sarah might have been a widow. I'm not sure that they were noting if any of the brides and grooms were widowed in the Withyham register. There is scant information in these marriage records pre 1754. Having said that, I haven't seen any other sign that this might be the case.
John