That endpoint may be used as part of very distinct flows, but if the flow involves an end-user which would mean you would be using that endpoint to exchange authorization code, refresh tokens or user credentials for tokens then this would not count for machine to machine quota.
What counts for the M2M quota would be a client credentials exchange (Client Credentials Flow) performed through that endpoint and for which an access token for a custom API you defined in the APIs section is issued.
The above means that for example, even if you do a client credentials to get an access token for the Auth0 Management API this would still not count for M2M quota as the Management API is considered an internal API.