Your argument is that you know better than CISA? You didn't even read the linked article properly. These were not recommendations for "regular work", they were

Your argument is that you know better than CISA? You didn't even read the linked article properly. These were not recommendations for "regular work", they were for "highly targeted individuals". But let's assume you're right and it's bad advice. The objection in the OP isn't that there was a breach. It's just exploiting the perception that using Signal is somehow wrong, and suggesting that it is a sign that Hegseth is incompetent. So it's rather important to know that using Signal is recommended by the government's own experts (even if those experts were wrong, hypothetically).