Besides, many people think of the term "progressive" in a very singular way. It's not (just) about the music being a progress from the music that was before.
It can also means the music progresses instead of going around in loops of "verse, chorus, verse, chorus".
I've always divided "progressive" music in two three camps with some overlap:
"Prog" rock/metal, which tries to emulate the sound of 70s Prog Bands (Genesis, Yes, Rush, Floyd, etc.) or 90s Prog Metal Bands (DT, Opeth, PT, Meshuggah, etc.) without really adding anything new to the genre. Most of the "lesser known" prog bands fall here, as does a lot of Djent.
"Progressive" rock/metal, which tries to break new ground while not necessarily emulating the 70s or 90s prog sounds. Stuff like King Crimson, Yes, Dream Theater, The Dear Hunter, older Muse, Radiohead, etc.
and "Art" Rock/Metal just aims to make albums/songs more thematic/connected/cohesive, or attempts to use classical or jazz theory in a traditional rock/metal setting. Art rock/metal won't necessarily try to get rid of the "Intro - Verse 1 - Chorus - Verse 2 - Chorus - Bridge - Solo - Chorus - Chorus - Outro" structure, but it'll do more thematic and interesting things with that structure. Things that fit here are older Muse, Radiohead, crow-era Incubus, Queen, Pink Floyd, The Beatles' later albums, My Chemical Romance's The Black Parade, etc.
Since there's overlap, something can fit in one, two, or all three subgenres. Pink Floyd, for example, fits all three. Dream Theater mostly fits the first two. MCR's The Black Parade isn't prog/progressive, but it's definitely art rock. And so on.