Can I install/convert a Giant Quick-e+ 2019 to use an Alfine 11 speed IGH?
Seems like a perfectly reasonable project, but there is possibly a technical implementation issue.

The SG-S7051-11 hub is part of Shimano e-bike series that includes a motor, and as you pointed out Shimano says the hub is designed for an e-bike, so it obviously is fine to use it on your Giant.

The dropout spacing offered 135mm only, I would expect the Giant to have that spacing as it has a disc brake. You will need to run a chain tensioner obviously, but that's no big deal.

The difficulty I see is the compatibility of the anti-turn washers with your dropouts. The installation manual here shows quite deep vertical dropouts which the Giant does not look like it has. I believe that is why you were told you need a torque arm. Finding one compatible with the Alfine hub may be a problem, as I believe Shimano does not make such a thing for that hub.