I think you misunderstood the claim: say the U.S. government leaned on the .com DNS server operators to issue a different response for, say, Gmail.com to certain requesting IPs. The absence of a mechanism like CT makes that very hard to detect since everyone else in the world is going to see the same correct response, and there’s no reason for the target’s DNS resolver to question a response with a valid DNSSEC signature, and since DNSSEC has no UI there’s not even a way for the user to notice.

That matters because, as the person you were replying to explained, there’s no plausible way to build such a thing. We have CT because the browser developers insisted on it and they control the clients but DNSSEC doesn’t have an equivalent party with that kind of leverage.