In my experience, it is much easier to say that I am uncomfortable when someone checks in on me than it to say it in the moment when something uncomfortable happens. A check-in is vastly easier to respond to, is usually predicated on care (someone sees that I'm not acting like usual and takes the time to say "are you alright?"), and can be done privately so I'm not put on the spot. There are also ways to do check ins that can involve group consensus around something. You can say "hey, I think this topic might be a bit rocky. What are we feeling about this?" and have people give 👍 or 👎 based on what how they feel about it.
But to your main point, all safety tools are shorthands or replacements for direct communication and that is why they exist. If no one needed a safety tool to communicate their needs fully, then safety tools wouldn't exist or no one would use them. They're frameworks in which rules about how to communicate are established - some safety tools give an outline for direct communication, such as the Luxton Technique - but that's not necessarily what a person needs in the moment. Some people have needs that are best supported by a tool like the x-card, and for some people the x-card fails to support them.
Sterling is expressing how the x-card fails to support their needs in the moment, and the specific ways it fails to support them. There are absolutely tools that exist that could support them better, and that failure is indeed directly the fault of the x-cards design. That doesn't make the x-card useless in all scenarios, but it also doesn't make Sterling incorrect about their own needs either.