Sadly, in an attempt to maximize profits and minimize efforts, Topping seem to have a tendency to cut corners everywhere, in product design, in support and in documentation.
If they don't use a proper subtractor/rebalancer circuit right at the input of the B100 it simply does not comply to most basic requirement of balanced connections: you must not make any assumption about signal symmetry, or in other words, common-mode voltage shall be ignored and shall not have any impact on performance.
Calling this an amplifier with balanced input and 100 Watts is a total scam, it's a shame.