Buy back is also a kind of reduction of share capital which is paid up and not diminution of share capital. But usually we call it as buy back coz it is defined in law by that name and with different section 77A.
The purpose of Section 77A and 100 is same but they are two different routes and law treat them differently. The only case of diminution in law is section 94(1)(e) as mentioned above for cancellation of unpaid capital only and not paid up capital.
If you reduce paid up capital by any method (77A or 100) that would not be diminution of share capital.
Regards