Yes, the Canon EOS R5 Mark II is compatible with the Delkin Devices POWER CFexpress Type B 128GB.
Canon EOS R5 Mark II accepts CFexpress Type B in Slot 1. Delkin Devices POWER CFexpress Type B 128GB has a manufacturer-rated 805 MB/s sustained write speed (no official VPG certification for this SKU) -- clears the camera's 30/60/90 MB/s floors for 4K30, 4K60, and 4K120 with wide margin.
“RAW Standard RAW 29.97 CFexpress 2.0 Type-B [400MB/sec. or faster]”
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Compatibility inferred from spec rule (citations above are manufacturer-verified)