Your theory is absolutely absurd. ZCash has been on the market for a long time now, and any backdoors would have been immediately noticed by other contributors.
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.
Maybe you can explain it to us then.
Right: This is a "possible backdoor. ZKsnarks technology required a kind of "key" to "start" the network. If you look to the history of Zcash, they really knew how sensible this topic is and spend hundred thousands of dollars to make the "start procedure" safe (in easy words: they split this "starting key" so that all team members have only fragments - and they involved civil law notary. The "key fragments" were later on destroyed).
So, yes, this is a possible backdoor - but the ZKsnarks technology requires this. I think they took care that everything runs safe.