Very, very few people truly understand how Zcash works. Even Zooko can't explain it. A backdoor could be something as simple as a vulnerability that they either accidentally found, or intentionally placed, and then kept secret. It's not like there's a big "THIS IS THE BACKDOOR!" comment in the code. It would be an extremely obscure vulnerability, that could then be used to deanonymize transactions. Considering that the founder essentially admitted there are ways for them to make Zcash useless for criminals, it's far from absurd to say that Zcash has backdoors. To me, it's practically 100% certain that they're there.