As far as I know, "Full Stack" is a misused term for developers who are able to do everything from devops to front-end development/ui/ux design.
Why would you say this? It's incredibly offensive, and adds nothing to your argument. A full stack dev might not be as good as a specialized dev in any of backend / devops / frontend, but that's just the price you pay for being a generalist. That doesn't mean they 'pretend to be able' to do things.