That reasoning is dubious.
They aren't saying that more have been hacked, they are saying that more have been discovered related to that hack. Any adversary at this level would be monitoring the news, and would take appropriate actions (for gain) or roll up the network rather than allow reverse engineering of IOCs.
More than likely this was not an LI based attack, but rather they don't know for sure how they got in. Nearly all of the guidance is standard cybersecurity best practices for monitoring and visibility, and lowering attack surface with few exceptions (in the CISA guidance).
The major changes appear to be the requirements to no longer use TFTP, and the referral to the manufacturer for source of truth hashes (which have not necessarily been provided in the past). A firmware based attack for egress/ingress seems very likely.
For reference, TFTP servers are what send out the ISP configuration for endpoints in their network, the modems (customers), and that includes firmware images (which have no AAA). Additionally as far as I know the hardware involved lacks an ability to properly audit changes to these devices (by design), and TR-47 is rarely used appropriately, the related encryption is also required by law to be backward compatible with known broken encryption. There was a good conference talk on this a few years ago, at Cyphercon 6.
https://www.youtube.com/watch?v=_hk2DsCWGXs
The particular emphasis on TLS1.3 (while now standard practice) suggests that connections may be being downgraded, and the hardware/firmware at CPE bridge may be performing MITM to public sites in earlier versions transparently, if this is the case (its a common capability needed).
The emphasis on using specific DH groups, may point to breaks in the key exchange of groups not known to be broken (but are broken), which may or may not be a factor as well.
If the adversary can control, and insert malicious code into traffic on-the-fly targeting sensitive individuals who have access already, they can easily use information that passes through to break into highly sensitive systems.
The alternative theory while fringe, is maybe they've come up with a way to break feistel networks (in terms of cryptographic breaks).
Awhile back the NSA said they had a breakthrough in cryptography. If that breakthrough was related to attacks on feistel network structures (which almost all modern cryptography is built on), that might explain another way (although this is arguably wild speculation at this point). Nearly every computer has a backdoor co-processor built-in in the form of Trustzone, Management Engine, or AMD's PSP. Its largely only secured by crypto without proper audit trails.
It presents a low hanging concentrated fruit into almost every computation platform on earth, and by design, its largely not auditable or visible. Food for thought.
Quantum computer breaks a single signing key for said systems, acting like a golden key back door to everything. All the eggs in one basket. Not out of the realm of possibility at the nation state level. No visibility means no perception or ability to react, or isolate the issues except indirectly.