Yes, the limit is applied to active editing sessions only. However, there are two modes for viewer: “regular” viewer and live viewer. The difference is that in live viewer mode users are able to see the changes made to the document in editing mode, whereas in “regular” mode they can only see current state of a document.
Please refer to the documentation about viewing in our official API: Viewing | ONLYOFFICE
You can find a note at the end of this article that says:
Please note that the server connections for the live viewer are counted separately from the total number of simultaneous connections and limited with the connections_view parameter in the license. For the open source version, this limit is 20. The live viewer is not available for the older licenses.