Hi Vince Legendre,
I tried to access this WFS service in the ArcGIS Online Map Viewer and I managed to add the layer Alluvial_Fans - see screen captures attached.
So, in the Map Viewer the Alluvial_Fans are visible, but trying to add them as a layer in ArcGIS Pro 2.5 I get this error message: Failed to add data. Bad address. Verify the server is running. So, my error message is longer than yours! :-):-):-):-)
So, I guess it is up to you to choose who to blame, the WFS service or ArcGIS Pro 2.5. I tend to opt for the latter...
HTH,
Egge-Jan