Donna you might get the skirting fitted free the way you posted your question, anyway to answer your question yes this person is right you never mitre internals they should be scribed in.
This is done by fitting the first wall with the skirting butted tight to the wall then cutting a mitre to the next adjoining length then using a copping saw cut the shape off from the face of the internal mitre.

Now this skirting will fold around in shape with the first peice you fitted.

Hope that helps

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