Presumably you meant Bruce Schneier. I don't think he is advocating for backdoors, what makes you think so? He is just advocating against cryptocurrencies, as he laid out in his article linked from the open letter: "Cryptocurrencies are useless. Blockchain solutions are frequently much worse than the systems they replace."
From Bruce via Don (see below), he did not intend to imply that recoverable credentials are a critical component of "safe" systems and he has other reasons for not supporting cryptocurrencies as evidenced by the writings you and Don linked.
Other than that, I just want to point out that I didn't say he was advocating for backdoors; I said he was indirectly doing that by way of the argument he had offered in the tweet supporting his post. Perhaps a better way of raising this concern would have been to ask the question, "Does Bruce believe that unrecoverable credentials prohibit the creation of 'safe' systems?"
https://www.wired.com/story/theres-no-good-reason-to-trust-b...