As good as ERG mode, it’s not instantaneous. It’s possible to hit spikes on that depending on what the rider does. It’s not like there is an absolute direct, 1:1 that standing equals a bad Ramp test. But considering the way and reasons that people stand in most instances, those would lead to potential issues with this specific objective.
Sure, it’s possible to stand and ERG can work to balance a bit. But the reality that if you stand and pedal at the current cadence (say 90rpm in a typical case), it is quite fast and “spinty” feeling compared to a typical standing cadence around 60-70rpm. Or you drop cadence (either by slowing and letting ERG adjust resistance, or manually shift the gears to get a more appropriate cadence without the ERG lag issue) and get a more typical cadence.
Or, as it appears above, the rider was already way down on cadence (mid 60’s) and stood, presumably to overcome mounting fatigue, and we see a big spike. In any event, I don’t consider ERG as some preventative measure here. Not everyone has or uses it for one thing. And it’s not a cure all that locks people into “perfect” power like many assume.
Essentially, I think it’s fair to state that standing should be avoided as it’s not what the test designers had in mind. Simple as that.