Jethro Tull certainly had hard rock elements in their music and hard rock dynamics in their live shows - Anderson would always complain that "to rock up" their act was expected from them on the US arena circuit -, but calling them heavy metal was perhaps ever so slight an overstatement .
I remember an Anderson announcement at a televised rock festival where JT were lumped together with ZZ Top, Status Quo, Joan Jett, Heart and Saxon in 1982. With the exception of Saxon none of them really a heavy metal act, but all offering a more immediate brand of rock (pre-AOR ballads Heart were perhaps closest to what Tull did, especially as regards the acoustic content). When JT did a few acoustic tracks, Anderson pointed to a mandolin and quipped sardonically: "
Alas!, an innocent mandolin at this heavy metal outing? Bear with us, it's only a little guitar, but - deep breath - it does its best!"
But the JT live experience (it was the very theatrical Broadsword tour with the pirate ship on stage etc) could hold its ground even before a largely hard and heavy rock audience. There was enough for the denim brigade to latch on to plus Anderson is a captivating front man. There is a reason why a lot of heavy metal musicians hold JT in high regard and the sometimes convoluted meters and highly structured song parts have left their mark on bands as diverse as Iron Maiden, Dream Theater, Opeth, System of a Down and even Slipknot.