The term for music without words is instrumental, not orchestral. If I play guitar and keep my mouth shut it's definitely not orchestral music. :P
If you look at the musician credits for the non-orchestral music, there's violin, cello, and flute, so I can understand thinking it sounds like an orchestra playing since it uses orchestral instruments. But there's no brass or several musicians playing the same instrument as there is in the orchestral tracks. Also, there's no mention of a guitarist playing along with the orchestra as it does for Martin does on piano (and my ears agree there is no guitar on the orchestra tracks). But there is guitar in the non-orchestral music, so that's a great indicator of which classification of music you're listening to.
There's also a
video discussing the making of the music in which Martin says, "we actually had to commission a whole orchestra to do the boss fights," indicating the smaller-scale involvement as opposed to being on every track. I haven't played the game yet, but, with the exception of "Victory", I think all the other tracks I named sound like boss battle themes.
But I wouldn't mind corroboration from someone else either; I merely went to Bandcamp and listened to the album, marked which ones were orchestral as I went, and did a quick re-check before posting.