I'm beginning to pick up a few things that (I think) need some correction and am not sure if this is the correct place to set them right !
Apologies if this is inappropriate.
I could be wrong, but for those who are studying -> I think you may have missed
After the 40 minute mark ...
(to enable the feature)
DISM /Image:c:\mount /Enable-Feature /FeatureName:TelnetClient
(to check that you enabled it)
DISM /Image:c:\mount /Get-FeatureInfo /FeatureName:TelnetClient
(then you can dismount)