From the outset, the forcing to the modeled behaviors of ENSO and QBO were split into a few categories. There is the declination of the lunar cycles, the proximity of the lunar cycles, and the sun's orbit. Each of these categories has similar complexity in terms of characterizing the orbital cycle to enough detail.
As it turns out, the declination forcing for ENSO, when isolated on it's own and retaining the 2d-order detail, provides an almost exact match as a QBO forcing.
The anomalistic cycle due to the ellipticity of the lunar orbit does not impact QBO like it does ENSO. That extra factor leads to much of the complexity of ENSO, while QBO is deceptively simple. It's deceptive because the 2nd-order details need to be right to capture the variation of the QBO's rather periodic pattern, and the ENSO model fit appears to parameterize this variation quite accurately.