I was going based off the commit link posted further down the thread (https://git.tartarus.org/?p=simon/putty.git;a=commitdiff;h=c...). You're right that the PuTTY deterministic nonce generating code does appear to significantly predate the RFC, but it sounds like the developer made a conscious decision when 6979 came out not to switch from what PuTTY already had because they looked similar enough. The PuTTY release that introduced support for elliptic curve cryptography (and introduced this vulnerability) was 0.68, which came out in 2017, 4 years after the RFC.
You're right that this was not an RNG security problem and instead was a problem with not understanding ECDSA nonce rules. However the notable fact for me was that the developer was apparently aware of the recommended way to deterministically generate nonces prior to the vulnerability being introduced and made a choice not to implement the RFC because what PuTTY was already doing seemed close enough, without fully understanding the implications of doing so. To put it another way, understanding ECDSA nonce rules would have avoided this vulnerability, but so too would implementing the RFC recommended way even if the developer did not fully understand why it was better than the existing implementation.
You're right that this was not an RNG security problem and instead was a problem with not understanding ECDSA nonce rules. However the notable fact for me was that the developer was apparently aware of the recommended way to deterministically generate nonces prior to the vulnerability being introduced and made a choice not to implement the RFC because what PuTTY was already doing seemed close enough, without fully understanding the implications of doing so. To put it another way, understanding ECDSA nonce rules would have avoided this vulnerability, but so too would implementing the RFC recommended way even if the developer did not fully understand why it was better than the existing implementation.