Look at Bob's link and it will show that some engineered wood products are approved for underlayment under the right installation methods in the TCNA handbook. (Only Table B is recognized even though Table A is listed. Remember, this is an APA-Engineered Wood Associations publication.)
Guess OldHunter couldn't find it and or is satisfied doing what someone told him he/she could do.
That, I guess is the difference between us. You clearly state "I really don't care one way or another". Well I do. And while I'm not in the Woodworking trade, there are thousands of Plumbers, Electricians, Woodworkers, etc. that have been doing things incorrectly or in a low quality manner because that's what they were told they could do.
As long as Menard's Baltic Birch meets the minimum thickness, tensile & deflection strengths, is T&G, and Exposure 1 rated.....then it can be used.
If it is not. Then it should not be used.