From reading the ASAN diffs it seems implicit interposing is the ability to use dyld interposition without requiring DYLD_INSERT_LIBRARIES i.e. implicit interposing works by linking to the dylib that contains the interposers. Previously ASAN runtime would re-exec the program with DYLD_INSERT_LIBRARIES and this is no longer required with implicit interposing.
Thanks for the pointer. I missed the part about ‘implicit’. This is a great addition.