Avman
The BEA report (still only available in French) deals with that point quite specifically. The United crew stated that they were stable for 9L, had clear visual to both 9L and 9R, were well-used to late sidesteps, and had no real concerns when they got the (erroneous) instruction. Worth restating that United then saw the Easyjet incursion onto 9R and initiated a go-round a couple of seconds before anyone else intervened. However, BEA note that no sidestep option had been briefed for landing, and they query whether United could truly be described as stable for the eventual 9R landing attempt.