Carbocation rearrangement in alkene addition reaction
The mechanism shown is not a carbocation rearrangement. Those two structures are resonance structures, which means that the chloride anion can attack at either of the carbons that bear a positive charge. Because there are two different alkene products that are possible, the more substituted alkene will be favored. Strictly speaking, you're right, Zaitsev's Rule doesn't apply here, because this isn't an elimination reaction. However, the reasoning is the same: the most substituted, most stable alkene is the favored product.