Lock in
What the idiot fails to realize is the reality of lock in. When you chose an OS you have lock in. When you chose various applications you have lock in. When you chose online services, again you have lock in. Now the issue is how severe is the lock in. Or as a question how easy would it be to migrate to different OS, application, service, etc.? If it is relatively easy then lock in is not a major issue. If there are significant hurdles (such as proprietary file formats) then lock in is a very real problem. So the question becomes not whether one gets locked in to AWS, you do but whether a competent person could migrate to another competitor relatively easily. Now remember one of the problems with any migration is not really lock in but the complexity of the system being migrated. Migrating a complex database (pick the one you like) is not a trivial undertaking even with the weakest lock in.
My personal needs for online services are limited and migrating to another provider would be fairly straightforward as there is nothing complex about my situation. But it would be tedious because of the amount I would need to move. So inertia keeps me put which is a form of lock in; the competing services are not significantly better that it is worth the effort to move. Now I can imagine if someone has a large, complex database with a good bit of infrastructure migrating from or to AWS would be undertaking and it the ROI had better be there. So if you are on AWS or Azure you might not be tied to the service that tightly in a technical sense but in a commercial sense you might be tightly tied; it's just too bloody expensive to move without a compelling reason.