Hi Rapid,
Having looked at many different webforums about can-packing (nned to do it myself soon) I'd be very confident in saying that the tape has been used purely to keep the packing material tightly bound as the sleeves are slipped back on.
The packing is what keeps the heat away from the outer shell of the exhaust (as well as dampening sound levels) so the tape really has the one purpose.
Most people use wire to hold the packing in place so tape to me, is just another option. The fact it hasn't burnt away is more likely to be testament to the packing material's heat dispersion than it being NASA spec fire proof tape
.
Guy