At least two errors. One
Divide litres of O2/min by bodyweight in kg (pounds ÷ 2.2) to get mL/kg/min
You’re off by a factor of 1000 here.
Two, you’re using powermeter output and not accounting for metabolic efficiency. Usually this is ballparked at about 25%, so you need to multiply by 4. This gives about 4.2 METs for your 100w 170lbs. It appears the standard for defining METs (17.5W = 1 MET = 1.22kcal/minute for a 70kg person) assumes an efficiency of more like 20%, which is the rest of the difference.