It sounds logical but I must admit that this is first time I hear it's independent of altimeters setting. I checked my 737 manual, some training material and some tcas stuff I've got from various sources on the net and I still couldn't find any reference to 1013.25mb. It says transponder must be operative for tcas to work, and correct me if I'm wrong, transponder is giving altitude based on present alt. setting (1013 or 0999).
Anyway, what I don't know - I don''t know. However I would appreciate very much if you give me some reference to read. Thanks