I don't think the average player would consider what you're talking about to be "competitive," though you're trying to make it sound like such. Yes, if you challenge another player to a leveling competition and they "cheat" by boosting to 90, you
could consider that P2W.
That's a really, really specific scenario, though, and there are already plenty of things that unbalance it, like recruit-a-friend perks, heirlooms, and death knights.
And it doesn't take that long to level to 90 anyway... A player did it last year in 12 days /played...
without killing anything.
Even in the oddly specific argument of "if we both created our characters at the same time and we try to get to max level and start gearing up," the boost only offers the most marginal of advantages, assuming you actually take the "competition" seriously. Depending on how lucky you are with drops, and how dedicated you are to playing (assuming 'very' since you consider this scenario 'competitive'), you could very well gear up before they did, even if they boosted to 90.
Besides, by your definition, basically
every MMORPG out there is pay-to-win, from EverQuest to Guild Wars 2. Pretty much every MMORPG offers some sort of paid leveling enhancers, ranging from EXP potions or gear packs to explicit level boosts or the ability to purchase "heroic" characters that start at higher levels.