Nothing is binary, everything is a tradeoff. Her texts didn't mention specifics so presumably anyone eavesdropping wouldn't have been able to identify the exact context. It's reasonable for high level officials to weigh the cost vs benefit. When you're one or two steps away from the president, you have a lot of leeway.
Yeah, you don't discuss war planning over an insecure phone. The OP talked about "in this situation", so no, you don't do that. And at least one member of that Signal chat was in Russia.
When you are one or two steps away from the President, you should be even more careful.
