Hi there and thank you for reaching out! I can see how this may have been confusing! This is the full context: 

"The months of May and early June are the best seasons for finding ripe, fresh and high-quality leaves. Picking them earlier in the morning is also optimal because, at this time, no grapes will be present on the vine."

Meaning, that according to the University of California Cooperative Extension, spring is optimal for harvest as grapes are not growing yet, and the best time to pick them is in the morning. Harvesting the grape leaves at the right time is important for the quality, flavor, and texture. During the spring months, the leaves will still be young and tender. Picking them in the morning just after dew evaporates and before the heat sets in will provide the best quality leaves.

Here is a resource for further reading: https://sacmg.ucanr.edu/files/266942.pdf