But what about the error in all other situations. Such as going alone against a strong wind?

There are at least competitors who do have a wind sensor.

The Sigma is just a ploy. Price doesn't matter much if a device isn't even in the race.

If the goal is feedback for training, then a Polar pulse meter with height meter (so evaluating ascent/descent) or a pulse meter with a GPS will manage just as well - even if they don't have menu setting to display "power". What they can do is compute the intensity based on pulse. Or based on speed in relation to height difference. And the second thing is basically the same as the Sigma displays. It's just a question of adding in a constant to translate from a fictive training intensity into a Watt measure.

But any system that does not contain real sensors will fail badly with wind. And wind is the #1 factor for biking. With the exception of mountain climbs, we would move around like rockets if it wasn't for the wind resistance. At 30 km/h, you have about 80W wind resistance. At 40 km/h, it's about 180W. At 50 km/h it's about 350W. Actual figures obviously depending on true size of rider, how aerodynamic the gear is, seating position, ... And ignoring all other losses. Friction tyre/ground, or in bearings or chain. Or the amount of energy needed to go uphill.

But the interesting thing is of course how quickly the wind resistance increases with speed, which makes a simulation like the Sigma fail badly if there is a 10 km/h headwind or tailwind. At 40km/h it would overestimate the power with 100W or underestimate it with 170W. If just having 2m/s wind, the error would be about -80 to +120 W at 40km/h.

What makes this even more important? How would the Sigma know if you are riding alone or in the front of the pack. Or if you have a comfortable ride behind someone? The difference is huge.

So I repeat it again - the Sigma is just a ploy.

If basing the training on the pulse measurements, you would at least get a direct feedback from the wind or where you are in the group. The goal with power meter must be to get a better feedback on amount of work done. Not worse.