This is true, but the bug here is not subtle. It would have been shocking to discover an ECC implementation that let attackers specify curve parameters even 10-15 years ago. When we blogged the e=3 debacle back in '07 or whatever, we linked to a Kaliski presentation from 1999 that called out validating curve parameters.
At least with the ACME vulnerability, there was a novel service model involved. Here, we're talking about certificates that allow you to embed what is in effect your own cryptographic algorithm to interpret the certificate under!
This is a rare instance where I'm happy to concede that closed source allowed a terrible bug to survive far longer than it would have if nobody needed to break out a copy of Ghidra to read the code that validated elliptic curve signatures.
Most people implementing ECC signatures are going to end up only handling a chosen group of named curves whereupon there aren't any curve parameters to check.
For example in Signal IIRC they use Curve25519. So there isn't any code about parsing or checking parameters, it's just here's some code that implements Curve25519.
NSS (the library used in Firefox on every platform) only accepts NIST's P-256 and P-384 named curves. It is parsing an ASN.1 data structure which can have parameters for curves, but if parameters are present instead of a name NSS's code gives up immediately because it's only looking for those specific names. (These aren't actually human readable names, they are OIDs such as 1.2.840.10045.3.1.7 for curve P-256)
At least with the ACME vulnerability, there was a novel service model involved. Here, we're talking about certificates that allow you to embed what is in effect your own cryptographic algorithm to interpret the certificate under!
This is a rare instance where I'm happy to concede that closed source allowed a terrible bug to survive far longer than it would have if nobody needed to break out a copy of Ghidra to read the code that validated elliptic curve signatures.