Maybe. And to be fair, their instructions are honest:

Simply wet Barburys Shaving Soap and use the brush to build a luxurious lather.

Nowhere do they specify that you should use the brush on their shaving soap, so I totally could wet this soap, and use Stirling. 🤔 I may get away telling my mother in law that I used the shaving soap following their instructions, and got great lather.