Safari is arguably the worst browser to do general tests in if you're not making iOS apps, as it has a history of rendering and behaving differently than any other browser.
The dev tools are also unusable compared to Chrome.
I think you misunderstood the GP's comment. They aren't advocating to work with Safari, but to work with Firefox then to test with Safari. If it works in Safari after being developed in Firefox, it's likely to work everywhere.
If you want to be sure your stuff works across browsers, test in safari
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