>Can you give me a source that enabling the topics API disables third-party cookies?
I never claimed that. The current estimated timeline for phasing out 3rd party cookies as of last month is:
23Q4 Opt in testing for sites
24Q1 1% disabled
24Q2 fully disabled
https://privacysandbox.com/open-web/
> And once Chrome has phased out third-party cookies the topics API will strictly decrease my privacy, not increase it.
You are free to disable topics, sites, or even the entire system altogether. While it does decrease your privacy in return you can get more relevant ads. With 3rd party cookies if you disable them all you will break things even if those things are unrelated to ads.