Administrative subpoenas are tenuous at best, but in the absence of an actual court order, words from ICE attorneys or officers saying "You are ordered not to disclose the details of this subpoena" have no actual weight in law.
The article pointed this out as well, but notably did not state that Google had in fact received an administrative subpoena.
This exactly. It's like everyone is assuming whatever ICE ordered Google to do was completely lawful. Even if this administration was a tightly run ship, when an agency gets a massive funding increase and daily quotas to hit like ICE did, all bets are off and you should never give them the benefit of the doubt. Obviously when the DHS secretary is calling American protesters domestic terrorists, cosplaying as a cop, and spending $200M+ on ads that feature herself, then you definitely give maximum scrutiny to everything that agency is doing/did.