That is Barr McClellan's surmise. LBJ spent a lot of time on his ranch or otherwise in Texas as Vice President, so plausibly, in the more relaxed security environment of the era, he had less need of Secret Service protection. Then again, it is hard to imagine LBJ giving them up as a component of his entourage unless he had good reason.
McClellan claims that LBJ gave a copy of the Secret Service presidential protection manual to his lawyer, Edward Clark, in order to plan the assassination. With that in hand, Kennedy's vulnerability to a sniper stood revealed, along with how to penetrate security and anticipate their moves.