As stated by Juho Sarvikas on Twitter:
Treble requires 2 conditions to be met:
1) Adapt to the new Vendor Interface implementation
2) Standalone Vendor partition to contain Vendor Implementation
Nokia 8 does not have separate Vendor partition in ROM space and the partitioning cannot be done OTA.
Link to the Tweet here.
Project Treble's main purpose seems to be to ease the distribution of updates. As Nokia was pretty quick with updates in the past, I don't know if Project Treble would lead to much improvement.
But Project Treble is also a major redesign of Android's architecture, and is expected to improve isolation between components and thus also overall security: