It is an idiom. I don't know why Duo does this, unless it likes to frustrate people.
Actually, I'm not so sure it is an idiom. It could be in some instances, yes, but if you live in Canada or The United States, there is a period in all weekdays between either 5 and 7 in the morning or 4 and 7 in the evening when everyone is getting back from work and school, which is called Rush Hour. :)
Rush hour is correct, it's just that we had been taught that ❤❤❤❤❤ = tip before now. With it translated as "peak" it makes more sense.
❤❤❤❤❤ = peak as in 'peak' hour. It's just that we haven't had this meaning before.
Ok I took it another way. ❤❤❤❤❤ we just learned was point/tip. So I thought 'hour pointer' thus hour hand for an analog clock (hour hand and minute hand for those under 25). yes I know mano is hand but I was trying to guess.