The purpose of the series if Bach was a cellist is to live the fantasy that everything Bach wrote, he wrote for the cello. It may also serve as a way for cellists to get close to Bach through works other than the six solo suites and the three gamba sonatas.
Bach did not write any cello concertos. Or did he? Many of his music manuscripts disappeared after his death. Included in that lost treasure may have been a couple of cello concertos. Bach wrote six suites for cello and three sonatas for viola da gamba, so, yes, why not a cello or a gamba concerto?