You need to at least Connect to the Spatial Portal, but not necessarily Sign In, to access the Public web services. If you would like to access the Restricted web services, you do need to Connect and Sign In to the Spatial Portal.
To Connect in ArcGIS Pro:
Click the Project tab on the ribbon, click the Portals page or click Manage Portal in the ArcGIS Pro map window itself
Choose Add Portal to add the Spatial Portal https://spatial.information.qld.gov.au/arcgis/ URL.
Right click on Spatial Portal and select Set As Active Portal:
Alternatively, on the top right hand side in ArcGIS Pro map window itself, Switch Active Portal.
To Sign In to Spatial Portal to get access to Restricted web services can be done when:
Adding the the connection via the Portals window
Already connected and using the Sign in link.