Safari can probably be discounted since Apple discontinued the Windows version in 2012. A browser that can only run on 18% of desktops worldwide isn't necessarily the competitor Google is looking for.

Edge is available on Linux and macOS, so it would probably do. But that would allow one of Google's biggest competitors to drop an under-performing product and lobby for antitrust against Google. Unlikely to happen, but a risk Google might not want to take.

https://gs.statcounter.com/os-market-share/desktop/worldwide

Chrome, edge and safari are just, well chrome, on a single family of rendering engines

Google would probably prefer if Safari and Edge did not gain market share. They're both developed by large corporations that pose a major threat to Google. Firefox, not so much.

Safari and Edge (which is also Chrome) having twice the dismal market share of Firefox doesn't make the browser market competitive. Safari is an appliance delivered exclusively on machines manufactured by a single company that holds a small, though luxury, part of the market. Edge (is Chrome, and) is only available on one OS (although granted, it's a very widespread OS.

Firefox is the only "credible" competitor, although Firefox's only profitable customer is Google itself.

It is plausible that it's part of a broader antitrust litigation defense strategy