Unfortunately, it is very hard to judge somebody's coding ability in discussion alone. You can sort of get the idea whether they have or don't have experience and whether they have luck being asked about topics they know (although you can help your luck just knowing a lot of stuff).
I have seen a lot of candidates who were quite smooth talkers but could not code their way out of a paper bag. Mind I do not mean remembering some complex algorithm. The task is usually some relatively simple data manipulation so that I can see the person approaching the problem, asking questions, getting involved in some discussion, etc.
The task is usually ambiguous a bit and this is explicitly stated so that the candidate is actually expected to get stuck, don't know things and have to talk to me to get the problem solved. You would be surprised how many candidates do not listen or can't follow simple advice even when I am almost giving them the solution on a platter.
The trouble is there is a lot of interviewers and many interviewers do not have a basic plan of what they want to achieve with the coding interview. What they want to learn about the candidates. Those interviews tend to suck.
What also sucks is candidates who come to the interview completely unprepared, unable to answer most of the questions or get any progress on the programming tasks and then spew misinformation on the Internet about how supposedly all coding interviews are stupid.
I guess if you can code you may have some misses, usually along with some hits. Sometimes you are out of luck. The point is not that you need to get every job that you apply to, it is enough to get one of them.
But if you can't code at all or you apply to positions that are clearly out of your league, you get rejected on all these interviews. And then what you do? Some people go to write on the Internet how all interviews are stupid without ever considering how they contributed to all those failed interviews, how real life works (ie. 90% of everything is crap including 90% of interviews) and how the situation might look from the other side.