No, welder's qualified under D1.8 would not be qualified under D1.5.

5.21 - Outlines that welders need to qualify under Part B of Clause 5.

Also, D1.8 is an additional qualification above the requirement of qualifying under D1.1.

There is also the requirement under D1.5 that you can not use power tools during welding of the qualification tests in D1.5 (5.21.5 Weld Cleaning). Not to mention if these are qualifications for performing Fracture Critical welding you would need to follow Clause 12.

I don't even see the "Engineer can accept other qualifications if he wants" line as in other D-codes, so I would be hesitant to even say that's allowed.