The WFS doesn't as you say 'prefer' the two coordinate reference systems cited, it only supports them.
<wfs:FeatureType>
<wfs:Name>CL:Cadastral_parcel</wfs:Name>
<wfs:Title>Cadastral_parcel</wfs:Title>
<wfs:DefaultSRS>urn:ogc:def:crs:EPSG:6.9:3812</wfs:DefaultSRS>
<wfs:OtherSRS>urn:ogc:def:crs:EPSG:6.9:4326</wfs:OtherSRS>
<wfs:OutputFormats>
<wfs:Format>text/xml; subType=gml/3.1.1/profiles/gmlsf/1.0.0/0</wfs:Format>
</wfs:OutputFormats>
<ows:WGS84BoundingBox>
<ows:LowerCorner>2.5299071005992668 49.484616315528285</ows:LowerCorner>
<ows:UpperCorner>6.4590126554566467 51.50583616059005</ows:UpperCorner>
</ows:WGS84BoundingBox>
</wfs:FeatureType>
By default it will use urn:ogc:def:crs:EPSG:6.9:3812
but otherwise you can request urn:ogc:def:crs:EPSG:6.9:4326
.
In QGIS 3 you have automatic reprojection, so if you have a project using the EPSG:31370 projection, then QGIS will translate the request into one of those supported by the WFS. If you want to use EPSG:31370 in ArcMap you'll have to configure something similar.