Amos Starkadder in fine bible-bashing mood, but possibly confusing the relationship between two Old Testament figures. Nimshi is described as the grandfather of Jehu and the father of Jehosophat, whilst Rehoboam was a King of Israel, the son of Solomon and grandson of David.
The suggested relationship is chronologically possible, although there does not appear to be any evidence for it. It may be that Stella Gibbons, who always had an eye for a comic twist, found the juxtaposition of the two names amusing. She may also have been aware of the other meaning of rehoboam: a wine bottle six times the standard size!
Alternatively, it may simply be that her knowledge of the Bible was hazy. Her father, Telford Gibbons, was a confirmed atheist, and she was educated at home by governesses until the age of 13. Stella Gibbons did become religious in later life, and was accepted into the Church of England in 1947.