Understanding the TTFB metric in Linkerd

[MA3CIN]

From the official documentation, the response_latency_ms "reflects the time-to-first-byte (TTFB) by recording the elapsed time between the proxy processing a request’s headers and the first data frame of the response".
So am i correct in understanding, that for two meshed pods, this measurements reflects this path:

Where response_latency_ms for outbound traffic for Service 1 is:
P1->P2 + P2->S2 + S2->P2+P2->P1
And the metric for inbound traffic for Service 1 would be:
P1 -> S1 + S1 + P1

Or am i getting it wrong?