I'm not very well educated in this space. Is DJB historically prone to exaggeration?
NIST P curves don't require any special preparation to their private key.
Curve25519 on the other hand requires a bizarre "clamping" (https://neilmadden.blog/2020/05/28/whats-the-curve25519-clam...), partially because the curve has cofactor 8.
Apart from clamping not being fully and clearly justified in most of the earliest specs and papers (which led to a few broken and/or insecure implementations), it makes the whole narrative of the curve being "safe" quite arguable and leaving a flavor of marketing in the mouth (NIST P curves have cofactor 1 so they are "safer" in that respect).
I don't think it's incompatible with a backdoor.
Why would they NOT backdoor it ?
Not that that's any guarantee with this administration, of course, but in general it's sound.
NBS became NIST and they still have credibility to you. Whatever reason you come up with why it was okay back then also applies today.
[1] https://en.m.wikipedia.org/wiki/Data_Encryption_Standard
The changes proposed made DES harder to crack for the NSA, but less "future safe" once computational power caught up. I don't know enough about DCA to know whether DCA-weak-DES could have been made safer with something like 3des, but I wouldn't want to bet on it.