The section reads as follows :

Number of Healthy Members is below
Sends an alert when a replica set has fewer than the specified number of healthy members. If the replica set has the specified number of healthy members or more, MMS triggers no alert.

A replica set member is healthy if its state, as reported in the rs.status() output, is either PRIMARY or SECONDARY. Hidden secondaries are not counted.

As an example, if you have a replica set with one member in the PRIMARY state, two members in the SECONDARY state, one hidden member in the SECONDARY, one ARBITER, and one member in the RECOVERING state, then the healthy count is 3.

The example does infer that arbiters are not counted but please change the middle paragraph to read :

A replica set member is healthy if its state, as reported in the rs.status() output, is either PRIMARY or SECONDARY. Hidden secondaries and ARBITERS are not counted.