So, is LTC set for a 2020 ATH?
Litecoin had its amazing rock months before its halving day. It is enough for wise investors to earn profits from that rally. All time high, a new one, in 2020, I don't think it will happen. Not only for Litecoin, but also for others altcoins. I believe that we won't see any new all time highs for altcoins (old ones, and except newborn altcoins) before bitcoin rocks.
Bitcoin, itself has never broken its all time high around its halving days, many times. I don't have any clue to be convinced that this halving bitcoin will break its all time high around $20k.
Back to the question, a new all time high for Litecoin, there will be one for it but it will happen much later, in 2021 or 2022.