One would think it unlikely that they only sell males, but.... They only sell males. Any site that says you sex dwarf gouramis (T. lalia - which is what you have) by their fins is incorrect. There are quite a few sources that spread this misinformation, unfortunately.
You can trust aqadvisor if you like, but I prefer to pay more attention to people's actual experience. You're not the first to try multiple dwarf gouramis (I've tried it) and you won't be the last.
By the way, the second link is for sexing regular gouramis, not dwarf gouramis. And the fist link clearly says that the females have much less color. It mentions dorsal fin too, but that's a moot point because the primary difference is a lack of color.
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