I know this kind of like the English word "sunglasses" where it's considered a plural, thus they used האלו. But can this also mean plural bikes? Because I got the answer wrong when I typed "these bicycles."
You're right, it should be accepted.
So what is the difference between one bycicle and many bycicles?
There's no difference. (I learned this from a native Hebrew speaker.) It's like moose or sheep in English.
I think, if you have more than one bike, you would speak of זוּגוֹת אוֹפַנַּ֫יִם ("pairs of bicycles").
hu yakhól leerót et ha-ofanáim ha-elu
audio for האופניים