“When a vocab word is all alone and has no okurigana (hiragana attached to kanji) connected to it, it usually uses the kun’yomi reading. Numbers are an exception, however. When a number is all alone, with no kanji or okurigana, it is going to be the on’yomi reading, which you learned with the kanji. Just remember this exception for alone numbers and you’ll be able to read future number-related vocab to come.”
Following that, if a number is all alone, then I have to use the onyomi (exception), but in the example* they give, the number ONE is not alone, it has hiragana next to it, so, why are they using the onyomi when the number is not alone?.
The only reason that ばん is written in hiragana there is because you haven’t learned the kanji for it yet. Those example sentences include ones that are readable with only your current knowledge, but they might be harder to parse than sentences written for adult natives. In that sentence, 一 is part of a compound, so the rule mentioned doesn’t really apply.
ーばん here is actually 一番, from level 8 vocab “Number one”.
Example sentences try not to use kanji from higher levels, so that you can read (pronounce) them more easily. Counting in Japanese is super tricky actually, so just keep moving. Do not try to get everything at once, it’s too overwhelming (speaking from very recent and ongoing experience
Notice that in the explanation, it says “when a number is all alone”, but it doesn’t say anything about the fact when it isn’t.
In fact, most numbers (the exceptions being 4 and 7) are almost always on’yomi readings, except when used with certain counters (and even then, it’s mostly 1, 2 and sometimes 3 that keep their kun’yomi).
Tofugu has a good article on Wago and Kango counting.
WaniKani example sentences are a bit weird, in that the easy ones have kanji you don’t know written out in kana.
The hiragana attached to 一 here are not actually okurigana, but a kana spelling of 番.
So, 一番 is a kanji compound, which makes it more likely than not that it will use on’yomi.