TCAS only requires the "other" aircraft to have a functioning transponder (of any type).
If the intercepting aircraft had only Mode A, then the 787's TCAS II will issue TAs (Traffic Advisories) as required
If the intercepting aircraft had a Mode C or Mode S transponder, then the 787's TCAS II will generate TAs and RAs (Resolution Advisories)
Only "coordinated RAs" require both aircraft to have TCAS IIs.