The agency are bang out of order here - it's not for them to decide not to disclose disability if that is what you have decided to do! They don't get to decide not to request the reasonable adjustments you need.
At least it was an issue with the recruiter and not the firm being discriminatory.
I hope they give you a new interview with the extra time 
About 7-8 years ago I had to speak to an agency after being assured by them that reasonable adjustments (in my case extra time for tests) would be in place for an assessment centre.
When I got there the woman doing the tests told me - in front of the other candidates - that she could only give me the adjustment after she had seen my statement of SEN from school. I explained I was diagnosed at 21 (the agency assured me this was already known) and so had no SEN statement from school. I said I could show my diagnosis letter but the woman said only a school age diagnosis could be counted and that I couldn't be 'that bad' if diagnosed as an adult and therefore didn't need the adjustment. Again, all in front of the other candidates.
I won in the end but it was with bad grace. The woman kept making comments about me having more time than the others including 'time's up everyone, except for Epi who gets more time than the rest of you for the same test'. She did this for both tests.
Afterwards she gleefully informed me that 'despite the extra time you still failed'. I told her there was no way I failed and asked to see my paper. She then looked flustered and revealed I hadn't failed after all, she was looking at someone else's paper 'by mistake'. Turned out I got the highest score. She then begrudgingly invited me to interview.
The agency were apologetic but said they had done their part by saying I needed an adjustment and were assured it would be put in place.
Every other company I've applied for has cheerfully allowed the extra time with no issue fortunately!