think this sounds fair, steam has channels to deal with the error, the fact there was a problem on steams part isnt a problem.
what he should have done if he was going to post on twitter at all is to point out the error and that steam was working to fix it. double whammy of getting his message across and subtly coaxing valve into doing it quickly [to make them look efficient at fixing things] without being confrontational.
if i walked into a bar and threatened to kill the barman, you can be sure he wouldn't serve me a pint [at the very least]
likewise if i walked into a bar and the barman threatened to kill me, you can be sure i wouldn't buy a pint.
as for steam having a monopoly, they provide a good service and [disclaimer]imo[/disclaimer] don't really charge anything more than the odds, certainly if anything i've both saved time and had convenience using steam compared to buying discs, so if they have a monopoly its a deserved one.
tl:dr
dev is impatient, makes stupid comment, steam cuts service to dev, seems fair