Running this code:
const response = await plexAPI.sessions.getSessionHistory(
'viewedAt:desc',
undefined,
{ viewedAt: '>=1723511329' }
);
Sends this url: http://<MY_PLEX_SERVER>/status/sessions/history/all?sort=viewedAt%3Adesc
So as you can see, the filter is completely omitted. The documentation doesn't say anything outside of:

The lib's source code defines the Filter type as an empty object:

The only thing helping to try and figure this out is inspecting the Network tab when going in Plex's history page:
