Google gave no details about their claim there would be national security issues.
The news author speculated that Alphabet will contribute fewer security updates to Chromium.
From my perspective, the core issue is the search and advertising monopoly and the drag on the economy. I’d like small business owners in future generations to be doing something other than selling junk on social media. And, enforcing antitrust law is a step toward that goal.
I’ll add that if Google really cared about national security then they would have started thinking about how to make divestiture a success years ago.