It has been a few years since I perused the writings of Melito in any detail. However, it appears safe to assert that he was a modalist, who thought Jesus possibly exhausted the reality of God the Father, Son and the holy spirit.
Jaroslav Pelikan (The Christian Tradition, volume I) alludes to the words of Tertullian, who argued that certain believers in or around his time "protect[ed] the 'monarchy' of the Godhead by stressing the identity of the Son with the Father without specifying the distinction between them with equal precision" (I:176-177).
Melito of Sardis was one such Monarchian, according to
Pelikan. He applied Psalm 96:10 to Jesus through
a so-called "christian midrash," whereby one
interprets the words "The Lord reigns from the tree"
as a polemic against Jews who opposed the glorious
Lord, Jesus Christ.
I notice that the Ecole Initiative states:
"The Peri Pascha addressed Marcionite teaching, even
if by extreme rhetoric. Jesus was not just prefigured
in the Hebrew scriptures, he was in the Hebrew
scriptures, suffering with the prophets, David, Moses,
Joseph, et al (415-504). Melito's graphic descriptions
of Jesus's death would be in direct opposition to the
docetic denial of Jesus' material body. Moreover,
Melito's modalism allowed him to affirm that God was
in all of these events and that he even suffered as
the person of Jesus. Marcionites must have found this
link to the Hebrew scriptures coarse and distasteful."