LSX is the code used in the timetable data for the Elizabeth line station, this code can be found in the Network Rail data under the column crs_reference_code but it is not publicised.

Similarly for PDX (Paddington Crossrail), FDX (Farringdon Crossrail), SPL (St Pancras Low Level), etc. The existence of separate codes, along with separate entrances / concourses / gatelines or even operators for ground truth both confirm that they are two physical stations for operational purposes.