I don't think you can be certain it's a newer Merlin engine OR that it will delay Block 5.

They do acceptance testing of all their engines, even the current iterations. They've launched literally hundreds of these engines, with multiple tests per engine (and, as an aside, a design for Falcon 9 that is able to survive an engine failure).

They test multiple engines per week, and have the capacity for multiple times that, so losing a test cell for a few weeks need have no impact on the schedule for Block 5.

My take is that this will have essentially no impact on SpaceX's schedule.