That particular quirk of the Kobo release zips is because they bundle a README and a koreader.png for fmon-purposes that *may* badly interact with (very) legacy fmon setups.
In practice, it's in 99% of the cases useless legacy cruft, which is why that's gone from the semi-auto packages

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TL;DR: As you found out, what you did should just work, as would directly extracting the zip (... in most every cases; certainly yours, as your device was pristine).
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Plato is another, younger document reader, while KFMon is a simple launcher that piggybacks on the native Library, instead of being a full-fledged app with an UI.
See the
wiki install page for a quick rundown of what this means in practice re: KSM vs. KFMon

.