What is the correct coverage area of EPSG:2901: Australia or Montana, US?

EPSG.io and spatialrefenece.com are third party web sites, for definitive information on EPSG codes you should probably use the official EPSG registry ~ EPSG Geodetic Parameter Dataset

Searching with 2901 on the EPSG registry shows us that the code is used for both a CRS

https://epsg.org/crs_2901/NAD83-HARN-Montana-ft.html?

and an Extent

https://epsg.org/extent_2901/Australia-138-E-to-144-E-8-S-to-12-S-SC54-onshore.html

In the EPSG registry, all geodetic parameters are codified, not just CRS, you can see from the search results parameters that have codes.

So if we look at the CRS NAD83-HARN-Montana-ft

We can see that the code for the CRS is 2901, that it is projected, that it acts on an Extent USA - Montana which has a code 1395

and if we explore further in the HTML page we can see that the coordinate system used is Cartesian 2D CS. Axes: easting, northing (X,Y). Orientations: east, north. UoM: ft. which has a code 4495.

It uses a BASE CRS NAD83(HARN) with code 4152

it uses the SPCS83 Montana zone (International feet) CONVERSION which has a code 15338

etc. etc.

The WKT2 gives us a summary of this too:

https://epsg.org/crs/wkt/id/2901

PROJCRS["NAD83(HARN) / Montana (ft)",BASEGEOGCRS["NAD83(HARN)",DATUM["NAD83 (High Accuracy Reference Network)",ELLIPSOID["GRS 1980",6378137,298.2572221,LENGTHUNIT["metre",1,ID["EPSG",9001]],ID["EPSG",7019]],ID["EPSG",6152]],ID["EPSG",4152]],CONVERSION["SPCS83 Montana zone (International feet)",METHOD["Lambert Conic Conformal (2SP)",ID["EPSG",9802]],PARAMETER["Latitude of false origin",44.25,ANGLEUNIT["degree",0.0174532925199433,ID["EPSG",9102]]],PARAMETER["Longitude of false origin",-109.5,ANGLEUNIT["degree",0.0174532925199433,ID["EPSG",9102]]],PARAMETER["Latitude of 1st standard parallel",49,ANGLEUNIT["degree",0.0174532925199433,ID["EPSG",9102]]],PARAMETER["Latitude of 2nd standard parallel",45,ANGLEUNIT["degree",0.0174532925199433,ID["EPSG",9102]]],PARAMETER["Easting at false origin",1968503.937,LENGTHUNIT["foot",0.3048,ID["EPSG",9002]]],PARAMETER["Northing at false origin",0,LENGTHUNIT["foot",0.3048,ID["EPSG",9002]]],ID["EPSG",15338]],CS[Cartesian,2,ID["EPSG",4495]],AXIS["Easting (X)",east],AXIS["Northing (Y)",north],LENGTHUNIT["foot",0.3048,ID["EPSG",9002]],ID["EPSG",2901]]