For GC135 - The online ballot was not 'a new mandate' to continue the strike, the original secret postal mandate to strike was still sufficient.
The online ballot was to see if the offer was acceptable to a majority of union members. Had the earlier immediately before the first strike offer been made available to union members, that would also have been done by online ballot, but BASSA called strike dates and that offer was withdrawn because they had done so. So, it's only ballots to call off a strike that can be online.
But yes, I agree, the fact that it is not a secret ballot could well affect the voting. 28% of eligible voters 'abstained' in the last online ballot, and could presumably be considered no voters if it were a secret one. But it is legal. The secret vote is only legally required for an actual strike mandate, not to call a strike off.