This isn't true. Crops with multiple harvests give experience for every harvest. (In the first paragraph under Experience Points.)
Maybe you're thinking of how crops that give multiple fruits per harvest (like blueberries and cranberries) only give Exp once per harvest? But if you plant cranberries at the start of the month, you will harvest them 5 times and get Exp every time. I leveled farming to 10 this way in my most recent playthrough.
It's not true. Crops with multiple harvests give experience for every harvest. (In the first paragraph under Experience Points.)
The stuff about sprinklers looks correct. But I find the lvl 6 sprinklers (Quality Sprinklers) very useful. I'm still using plenty of them on my farm even now that I'm swimming in the materials for the lvl 10 sprinklers.