The IEA ETP data used in the SDA only provides carbon data as far as I know, how was it transformed to overall GHG data?

Yes, this is correct. SBTi made a comparison between IEA emission reduction scenarios to WB2˚C and 1.5˚C scenarios in the Foundations of SBT setting paper (pg. 28) and concluded that targets modeled with the SDA using the B2DS scenario can be considered aligned with a WB 2°C temperature goal.