Meaning of the sloka
Ravana chose not to be killed by anyone except humans. Kumbhakarna wanted to surpass/eliminate all devas, instead asked for constant sleep, by slip of tongue and mispronouncing the word.
Kumbhakarna actually desired for ‘Nirdevathvam’(state of absence of devas or Gods), but ended up asking for ‘Nidravathvam’(long sleep) instead.
Ravana asked for the boon of never getting killed by any of the Gods, Goddesses, Devas, Asuras, Gandharvas, Yakshas etc. The only group he did not mention was humans, this was because he thought himself to be superior to any mere mortal.
There is a very funny story related to the boon obtained by Kumbhakarna.
Devas were sure of the innate desire of Kumbhakarna of eliminating all devas and felt helpless. They approached Goddess Saraswati and pleaded for help. Goddess Saraswati promised help to the Devas lead by Indra and while Kumbhakarna asked for his boon, she sat on his tongue making it difficult to say what he wanted as his boon. Adamant by nature, Kumbhakarna forced the words out of his mouth, but alas what he said was completely different from what he had envisioned. Nirdevathvam (Eliminating all gods including Indra, Agni, Vayu, Yama etc.) changed to Nidravathvam ( Long sleep).
In some versions of Ramayana it is mentioned that Ravana had to do penance again to reduce it to six months of sleep and six months of active life for Kumbhakarna rather than complete sleep for the rest of his life.
This story which was narrated to me in childhood taught me two things, slip of tongue can cost you greatly and second why there was a prevalent insult of ‘ he is like Kumbhakarna’ for people who sleep more.
My primary source for meanings is from the E-book published by http://hinduebooks.blogspot.com.
Om Namo Narayanaya