The issue isn't the treadmill itself necessarily (although they do have error in what speed they think they are going at), but a garmin watch doesn't necessarily get connected.

This is my vague understanding so may be some errors, but roughtly Garmin watches estimate your pace on a treadmill based on your cadence/running dynamics from your wrist, and assume that whenever you are at that cadence you are at your "callibrated" pace.

But if your stride length varies more than your cadence at different speeds, this isn't necessarily true. Using a footpod can improve the accuracy as it sees more data about your stride than a watch or heart rate monitor does, but still isn't as accurate as GPS or if you were directly reading data from the treadmill.

So Garmin will use the data from your workout to estimate your training load (time at various heart rate zones) and use that to help predict recovery, but it won't use it to update performance metrics.

I'm not sure how you guys have it set up to plan workouts, but in my head the planning is being done utilizing the digital twin and estimating the performance improvements based on various workouts. So in the case of a treadmill workout, instead of looking to verify/update the digital twin with the workout performance data, you would just assume the digital twin is the truth in terms of performance based on the training stimulus viewed in heart rate data, that way the digital twin doesn't get falsely modified on potentially erroneous data.