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This would likely be used on the outer surface of the pipe where the curvature would have minimal impact to the readings.

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Yeah, the description doesn't indicate if the probe's surface is convex or concave....

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The vendor tells me the probe in question is flat. Is a 6 mm (1/4") flat probe used w/ the couplant acceptable for sonic testing cylinder wall thickness?

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Brad, if you look at the face of that probe you'll see the center divide. I bought one like that and lightly radiused both sides on a very fine belt sander to get the convex i needed. I've tested mine on material thicknesses ranging from .100 to .500 and it's always within .001-.002 of what the mic reads. For sonic testing cylinder walls i think that's plenty close enough.

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I bought two, in case I screwed one up. I used 600 & 800 wet-or-dry in a 4.375" tapered ring compressor to gradually apply a radius to the face.

I tested both versions of the probe back to back and found I had to recalibrate the tester when switching. The radiused one provided much more consistent readings in the bores and seems to do the trick.