Also someone with good standing in the company to politely point to the CTO that it's not his job to give his 2c at every conversation in slack (typical behaviour of people coming from the technical side)
I find it very dangerous when you have some technical people who moved upwards into non-technical roles still being involved in technical discussions.
Their words carry a lot of weight, yet they have no idea about the actual context of the work being done.
Sometimes this is useful and a fresh perspective from an experienced colleague can unblock things, but more often it stifles discussion and discourages the juniors from thinking freely.
I've been on teams exactly like this, but the problem isn't public channels, it's "leadership" not knowing how to let go and trust their teams. Doing more work in private is the negative consequence of this bad practice, but I assure you this issue will eventually cause problems no matter how clever people are in avoiding public conversations.
And startups that are on the path to long term success will have leadership abandon this type of butting into every day communication very early in their growth. A good C-level must learn the build teams that can scale beyond their individual interventions. You literally cannot grow beyond a 30 person organization (and survive) if this much intervention is required.