Some can still code, and that’s your competition. Many cannot.
Most of the best developers I know would fail a technical interview for their current job.
But to be fair... is that an actual technical interview?
Yes, that's because they can't code.
(No, connecting APIs and navigating information complexity landmines isn't coding.)
In silicon valley software companies (e.g. Google, FB, Twitter, Stripe, Airbnb, Uber, and all the smaller companies following the same interview & career ladder playbook) a senior engineer or L5 is just someone with around 5+ years of experience. They might have a few more meetings and spend a bit more time working on design docs, planning, mentoring others, etc. compared to more junior engineers, but their primary job is still writing code. Even at higher levels like Staff and Principal engineers, less time might be spent writing code and more time in meetings, leading committees, approving the largest in scope design docs, etc. but there are so few of these people and the bar is so high to get there that they were at least at one time some of the strongest engineers at the company, and they still code sometimes even at this level. In the entire IC track at these companies, I suspect you'll find approximately 0% of people that are so incompetent they are just unable to implement fizzbuzz. In the management track you'd find some, but even there I suspect quite a low percentage.
In other industries, maybe banking, large contractors, more old fashioned megacorps like the GE's of the world, I think there might be more "architect" and other technical tracks that are allegedly still programmer IC tracks but where the senior folks are much further removed from programming and wouldn't be able to implement fizzbuzz.
I got asked this problem and my mind went completely blank and I had to solve it from first principals, making some of the same blunders I see others making when I work with them through it.
When I see people claim that senior devs can't FizzBuzz, I have to imagine what they mean is that they do some silly error because they are under pressure and you have to remind them about "what about 15?" or something. I've interviewed hundreds of developers and only two had trouble implementing a loop and conditionals as a concept.
Whiteboarding Yet Another Graph Search Problem With Perfect Syntax[1] might be beyond their current short-term abilities, but I'd be surprised if they couldn't produce pseudocode and architectural design that solves most real-world problems.
On the other hand, there are plenty of "seniors" who couldn't write code in the first place.
[1]Everyone knows which employer does this.