Was that definitely the case? I hasten to add that I don't know in the case of the Sea Vixen, but a straight tail-plane would not inherently suffer from compressability issues before the main-plane, if it had a thickness to chord ratio that was sufficiently lower. The greater lift curve slope of the straight tailplane versus the swept wing would help to maintain manouvre margin with increasing Mach number. I read through the article but couldn't see a reference specifically to the tailp-lane suffering as opposed to the main-plane.
I recall reading about it elsewhere some time ago, but I don't recall where. A straight plan-form can be sufficient for high speed flight if the fineness ratio is high - as in the F104. longitudinal stability at high speed is a favourite interview question - "why do airliners have greater sweep on the tail-plane compared to the wing?" (The 747 has the same sweep on both, but gets round it with a greater fineness ratio). But looking at a picture of the Sea-vixen, there does seem to be a bit of camber on the tail-plane. The tail design went through several changes and although the DH110 achieved supersonic flight in testing, in service it was limited to a very low M0.92 considering it's role.
Dan talks about pitch stability at the extremities of the range in his article and attributes much of this to elevator gear changing. But an ex-colleague who flew them mentioned the dreadful high speed handling of the Sea Vixen. It was so powerful, you could get to VMne very quickly and it was easy to exceed. My friend scared himself several times and remarked that he was glad when the Phantom came along as he considered it far safer and that he was 'on borrowed time'. If you consider the F4 to be safe, then that says something about the aircraft you were flying before! And the statistic of some 40% of the fleet lost in accidents also says something.