I checked:

• Socialist Workers Party
• Alliance for Workers’ Liberty
• Socialist Party
• Trade Unionist and Socialist Coalition
• Communist Party of Great Britain (Provisional Central Committee)
• Revolutionary Communist Party

and they dont seem to explicitly disallow dual membership. The vote was to allow dual membership with those aligning with the parties values according to the CEC. You are right that the decision does not break these terms. Unfortunately I think it does go against the spirit of the vote but that is entirely subjective.

I just think it is strange to push away supporters who largely agree with the party and want to help. Especially ones that explicitly support the mission.

Just seems a shame :(