That's not true. For an example, see Schneier's comment here:
https://www.schneier.com/blog/archives/2013/09/the_nsa_is_br...
Or Bernstein and Lange's comments here:
https://www.hyperelliptic.org/tanja/vortraege/20130531.pdf
(specifically the: Jerry Solinas at NSA used this to generate the NIST curves (or so he says))
I believe Matthew Green may have also made a similar statement, though I can't find it, so perhaps I'm not recalling correctly. In any case, I don't think you can outright say "No cryptographers believe the NIST curves are backdoored". You can at best say "No cryptographers have proven the NIST curves are backdoored", which is true.
However, those cryptographers have also raised concerns (including concerns about backdoors) and I just hope we move to safer alternatives quicker.