They haven't given any other such reactions or what would happen otherwise(such as BF3 first and AlCl3 later) orgopete. My study material isn't so deep, unfortunately. I think this must be a general case for all alkyl groups, as cation rearrangement does not influence the site of attack. It merely generates different products as per different positions of the positive charge, and the most stable carbocation ends up giving the major product.
My worry is regarding the sites of attack of the alkyl cations. Why this and not some other sites?