Interestingly it states that the M.2 slots support storage pools in the foot notes of the feature page:
Synology SNV3400 series M.2 NVMe SSD drives can be installed through the built-in M.2 slots to enable SSD caching or create SSD storage pools. Drives are sold separately.
Source: https://www.synology.com/en-global/products/DS923+
Synology SNV3400 series M.2 NVMe SSD drives can be installed through the built-in M.2 slots to enable SSD caching or create SSD storage pools. Drives are sold separately.
Source: https://www.synology.com/en-global/products/DS923+