I read through the interview questions at the link, and yeah, I'd say those are somewhat tough questions. I've been doing this for 30+ years. I've written a few books. I've maintained a blog for 15 years. I honestly feel like I've got some knowledge and experience for real. Yet, I have to look up examples on DENSE_RANK to be sure I'm using it correctly, or even that it's the right solution for a given query. That knowledge isn't just sitting in my head ready to be accessed.
The list of questions you have seem mostly reasonable. Maybe not the running totals one (again, I just don't have that info in my head) or the XML. The rest are, yeah, reasonable for a senior person to know.
However, I feel your pain. This is a list of questions I used to ask (well over 10 years ago now, some would be changed with more up-to-date info). As it says in the blog post, this eliminated about 4 out of 5 candidates. I also like open-ended questions where we can talk about how someone approaches problems.
Finding good people is frankly difficult. I think that's most of what you're hitting. Whether or not some of the interview questions are overly focused on specific T-SQL behaviors or not, yeah, that's fair enough. Heck, if I were interviewing, I'd say that outright, "What? You think that stuff stays in this tiny brain full-time? Nah, I have to look that up." But then I'd explain how I would look it up, test the implementation and then show it works. I'd also go into talking about performance implications, if any, and probably start trotting out execution plans. However, you wouldn't see me regurgitate a lot of really complex T-SQL on demand. Yet, I really do think I'm senior level.
Maybe I'm wrong. Wouldn't be the first time.