These are very good points but at least Apple could give the user a choice to report air quality consistently when traveling. At the core, this is not a measurement problem (the underlying data is likely the same) but it is a user experience issue since a user cannot use the color coding as a way to compare different scales across locations.
I think the problem is that the colors have different meaning. The values in the screenshot for Amsterdam using the NL scale translates to 'insufficient' and while the DE scale translates to 'good' for the same city at the same time.
At the very least, it means that a user cannot use these scales for inference without knowing/understanding the technology behind them. This is probably not the intention behind showing an air quality indicator in the iOS app.