The Ramayana, Mahabharata and Puranas are often said to be smriti. But I have seen several lists of smritis given in various Hindu texts and none of them include these Itihasa texts, they only include the Dharma-related texts like the Manusmriti, Yajnavalkha Smriti, Naradasmriti and so on.
For eg, here's the list of given by the Yajnavalkha smriti
Manu, Atri, Vishnu, Harita, Yajnavalkya, Usana, Angira, Yama, Apastamba, Samvarta, Katyayana, Brihaspati, Parashara, Vyasa, Sankha, Likhita, Daksha, Gautama, Satatapa and Vashishta.
The Parashara smriti gives a similar list but it replaces Yama, Brihaspati and Vyasa with Kashyapa, Gargya and Pracheta. And the Padma Purana lists 36 smritis; its list is similar to the list of Yajnavalkha but it omits Atri and adds 17 new smritis given below
Marichi, Pulastya, Pracheta, Bhrigu, Narada, Kashyapa, Vishvamitra, Devala, Rishyasringa, Gargya, Baudhayana, Paithinashi, Javali, Samantu, Paraskara, Lokaksh and Kuthumi
As we can see Ramayana, Mahabharata and Puranas are absent. The Vyasa Smriti written by Vyasa is mentioned in at least two of the lists but his more important contributions, the Mahabharata and Puranas, are absent.
Also I was reading the writings of Prof. Purushottama Bilimoria who is a fellow at Oxford University and he makes quite a grand claim about all the texts included in smriti.
"...The smriti texts comprise, in particular, the six Vedangas (the auxiliary sciences in the Vedas), the epics of the Mahabharata and Ramayana, the Dharmasutras and Dharmasastras (or Smritisastras), Arthasasastras, the Puranas and kavya or poetical literature, which regulate Hindu social order..." - Bilimoria,Purushottama, The Idea of Hindu Law (2011), Oxford Centre for Hindu Studies
So I'm curious is there a scriptural basis for including all these texts under smriti? Or is smriti just some open-ended category of traditional literature in which anything ranging from the Manusmriti to the Shiva Purana to even Kalidas' Meghdutta can be included?