Nah that’s low humidity that’s causing the leaf curl. Always an issue when you get a lot of light in a small space in warm weather. It’s more of an issue when they are small cos they cant create a decent micro climate until they get to a reasonable size. I spotted his humidity gauge in an earlier pic at 45% and when they are small like that they try to retain moisture when it gets too dry for them. They will still grow but slower and will be prone to other issues cos thye cant take up nutes properly, cant transpire water through leaf pores etc. They seem to love it when it’s about 60 - 65% RH at that size and a bit bigger.
I’m not being critical here at all cos the guy’s been doing a great job from what i can see and i have just recently been caught out by this same issue. Even after growing weed for well over 20yrs.
I should have added, for those who might also have this issue there is a great FAQ on Vapour-pressure deficit, or VPD right here on this most excellent forum 