Why is ICF with watertight concrete in not classified as a Type B basement waterproofing system?

Because it is not feasible to compact it as well as is required for water resistant concrete, plus all the ties required provide many water pathways.

Further to this, does it make a difference if external drainage is used? No, a waterproofing membrane would still be required.

For an example of a CAD drawing illustrating ICF, please click the following link: http://www.safeguardeurope.com/cad/G-XS-001.pdf