THIS happens ALL the time.
The entire Reporting Relationship/Team Dynamic changed.
That's a valid reason to move on.
Incidentally, might be worth connecting with the former CTO via Linkedin.
While he may be contractually prohibited from employing you in his new role. He could serve as professional reference.
Typically nonsolicit agreements only prevent the CTO from reaching out and trying to hire you away. If you, the disgruntled engineer, reach out to your former CTO and ask them if there's a job at their new company, the CTO will typically not be prevented from employing you.
It could happen.
And yet, relationships with Ex-C Level Execs can be oddly toxic. Even up to spiteful litigious situations.
The Ex-CTO may be receptive your call.
But likely he'll wait out a cooling-off period to avoid the 'appearance' of solicitation.