The Five Eyes is a coalition of intelligence agencies of Canada, Great Britain, The USA, Australia and New Zealand. Its an agreement to work collaboratively and to share intelligence in an open and collaborative way. The intent of course is that more "Eyes" working together with different frames of reference and geography around the globe sharing intel and resources and data helps all get the story straight.
Kinda like having a community watch group of concerned citizens each looking out for each other.
Long and short of this is that if Trudeau has made an accusation openly - that accusation will be backed up by intelligence from CSIS, MI6, the CIA and other US Alphabet agencies, and the intelligence agencies of both Australia and New Zealand.
This would not be just an accidental closing of a random synapse or a gut feel remark. There would be absolutely rock solid intelligence supporting such a claim.