You should read the release that CISA put out [0]. The use of Signal for classified discussions is not a suggested use. True, it's not explicitly forbidden, but people entrusted with that access should know better.

Saying that CISA approved Signal (and, in right-wing sources, saying "Biden administration CISA") is an attempt to minimize and distract.

They shouldn't have been texting classified information. Full stop.

[0] https://www.cisa.gov/sites/default/files/2024-12/guidance-mo...