blaziken without speed boost is as good as infernape (and some other fire-types you can get later on). that said, i wouldnt call it the best choice as a starter. it's still a great one given the first few gym leaders, but without speed boost, it's way more balanced than it is right now imo.
can you elaborate on how that is not true at all? have you tried doing a run relying completely on blaziken? its not something you can do with most mons, and when you can, they usually get axed right away (a good example would be gyarados). my younger brother, who's not very experienced in pokemon and occasionally does runs of fanmade games, decided to try out reborn a couple months ago, as i suggested him to. i warned him that it would be a really hard game comparing to what he's used to and that he would probably find it extremely difficult to get past most gym leaders, without at least coming up with a decent strategy and follow it through. i only said this because i had never tried to use mons like blaziken and gyarados, since they turn the run that boring. to my surprise, even an amateur like him got past most of the gym leaders on the first try and without giving his best at all. i would occasionally help him, but it would be nothing more than that. all because he got a speed boost torchic and relied on it that hard up until the end.
note that he did have other team members such as azumarill and mons you can get early on the game, though, those hardly matter. if you get a blaziken, the least you have to do to beat the game without any major difficulties are healing items, hm slaves, support/bait mons and a shit ton of common candies. the first few boss/gym battles are a joke if you've got yourself a combusken. even at such an early stage, it can sweep teams and it doesnt need much healing or support to do that. the support/bait mons come into play only later, especially on battles where you have to face mons that have type (and whatnot) advantages against blaziken. in singles, slightly weakening one or two mons that blaziken wouldn't be able to OHKO is more than enough to allow it to take care of the rest of the team by revenge killing said mons and spamming its stab moves while getting speed boosts, which, after one or two mons down, is enough for it to outspeed the opponent's entire team. another thing that's extremely easy to abuse is how you can predict whenever the opponent is going to use an healing item, and use that as a chance to set up a swords dance (or even bulk up, which it gets pretty early on) while getting a speed boost. in most cases, just one boost will be enough to sweep the opponent's team, and even if it isnt, you can always rely on the support/baiting mons you've got on your party to continuously heal blaziken, allowing it to deal even with mons that would normally counter it with ease. and let's not even mention the shitty mons that can do nothing to blaziken to begin with: those allow it to set up as much as it wants to, making it pretty much impossible to counter afterwards.
on doubles (for example, charlotte's battle), the support/baiting mons are even more hilariously effective. as you set up to sweep, they can either draw attention away from blaziken and die instead of it and/or you can use them to heal blaziken while he's setting up/sweeping. speaking of charlotte, my brother high jump kicked her entire team on his first try (as well as most of the other gym leaders' lol).
take into account that it can do all of this almost alone, only needing some support. now imagine doing a run with mons whose only task is to counter mons that threaten blaziken. im sorry, but if that isnt as easy as game freak runs, then i really dont know what is (not counting with gen 6 exp. share runs, that's even more of a joke).