That is a good question. Since it is more tollerent than either the parents or grandparents I would say that it was not a direct contribution but the result of the hybridization. This might take one of two forms. Either a set of genes that were not a significant factor by themselves combined into something significant, or a gene that was "muted" got switched on and became more strongly expressed in the shuffle.
I would lean toward the later. To put it another way, when genes are shuffled around in a hybrid to figure out what needs to pair with what sometimes a genetic switch is flipped that causes a gene to be expressed that had never been noticeable or active before. This is why unique flavors and colors (in flowers) pop up in hybrids that had never shown up in either parent. If it were a "synergy" of genes I would have assumed that most if not all Tangelos would be extra resistant to HLB. This is not the case, and tangelos are mostly considered susceptible to HLB (including Page which has the exact same parentage but was developed about 30 years before). To me this demonstraits that there is something special going on that is beyond simple Mendellian genetics.