The author is pretty confused. x264’s slower presets enable searching more points in ME and a more accurate RDO (among other things), just like x265’s. But in all cases for any modern encoder, all mode decisions are done baed on the estimated bitcost relative to distortion, so the slower presets are just evaluating more options. (kinda)

That said, it is the case across all codecs that more accurate motion vector fields (e.g. using optical flow) are not generally optimal. A simple way of reasoning about that is that one block reusing the same motion vector as a neighboring block is much cheaper than encoding a marginal difference, which often more expensive than the bits saved in the residual.

Anyway, CRF was never designed to map to the same quality across different encoder settings, which having actually read the article, is what I think they’re actually complaining about.