I actually took their response in a totally different manner.
It seems like they are frustrated because they put such a huge emphasis on interviewing skills. For example, take two candidates. Candidate 1 is an okay fit, but has great interviewing skills, and candidate 2 is a great fit, but does not interview that welll. Who would they take? It seems like they are frustrated because sites may place more emphasis on one day (interview day), as opposed to looking at the the "whole" candidate. The fact of the matter is that some people do not interview as good as others, but does that mean they would not be a good fit or do a good job? Do you rank someone higher purely based on the one interview day?
Obviously, this does not mean that because you interview well, you will not do a good job. There are A LOT of people who interview well who would do a great job. And there are A LOT of people who do not interview well that would do a great job.
That is how I interpreted their statement.