Read the article. This is an extremely small study (10 participants) and suggested that midazolam was also kind of effective, but even that's not really clear because ketamine and midazolam were both administered to the study participants.
Let's see, current "natural" remedies (competitors or complements?) for chronic fatigue to the proposed ketamine:
adaptogenic herbs (like ashwagandha or ginseng)
exercise?
coq10 / magnesium / b vitamins?
Ashwagandha is awful in my opinion: like Valium it relieves anxiety and makes you stupid but the ratio is worse.