I have indeed never sweated a copper pipe to a lead pipe - because I don't have the skill to do the job to a professional standard and there were modern alternatives available to me. I have seen many such joints (to iron and brass as well as copper) done by professionals - ancient and modern - all of which were "wiped" into a thick layer. And even seen a few done. I was told they leaked if the lead was not built up. But you may of course be right that it was unnecessary.