Yes possibly. If such centralized apps grow large and is a popular way to access nostr, a leak of one giant database containing nsecs could be a huge blow to nostr down the road.
Discussion
I think a much bigger threat is posed by a giant app centralizing access to Nostr feeds then starting to censor selected people. ZBD could do that, but I don't think ZBD will become this huge app capable of making such a negative impact.
I would like to see many other "custodial" platforms integrating with Nostr too to keep everybody in check while leaving open room for the actually open and independent decentralized clients.
That’s a threat too. But at least you have client choice