Is it asking to grab all TCP packets on byte offset 12 (TCP Header length and Reserved bits) with values at least 80 that is true
No. 80 in decimal is 50 in hexadecimal, so it's equivalent to tcp[12] & 0x50 !=0
, which tests whether either the 0100 0000 bit or the 0001 0000 bit in the 12th byte of the TCP header are set. That's true of 0101 0000, but is also true of 1111 0000 and 0111 0000, as well as 0100 0000 and 0001 0000 and 0100 1111 and....
If you want to test the uppermost bit of that byte, you'd use tcp[12] & 0x80 !=0
. That would, in effect, match all values >= 0x80.