Just because they spent money to help them win doesn't make the game pay-to-win.
While it's tempting to conclude that because they put money in one end and landed up with BoE equipment at the other end, that they paid money in exchange for equipment, it's actually a flawed viewpoint that lacks insight into the mechanisms at play. And just because pay-to-win is a mechanism that achieves that outcome, that doesn't mean that it's the only mechanism capable of achieving the outcome.
Method did not spend the money to buy equipment and corruptions - because neither Blizzard (through the shop) or the game (through a vendor) sells those things. Other players sell those things. And even the gold that you acquire with a token isn't give to you by the game, it's acquired from other players. They spent their money to buy gold that other players had obtained through time and effort. So a more accurate way of looking at this is that money allows you to buy another player's time and effort. And even without tokens, a guild like Method that has corporate sponsorship can "sponsor" other players to play for them and farm for gold.
The whole point of an MMO is to work with other players to achieve more than you can achieve alone. And by extension, the more players you can get to help you to achieve whatever it is you're trying to do, the bigger the advantage. Being able to use forces outside of the game to do this is part of the MMO experience. Streamers do it all the time by using their celebrity status to influence people. Some players are just charismatic and good and getting others to help them. And yes, sometimes players pay other players to do it. But the important thing here is that even if you're paying other players, your success is still predicated on other players do whatever it is that gets you the advantage. It's still people playing the game that results in the 'win'.
Pay-to-win, on the other hand, has nothing to do with other players. It's entirely about a transaction between a player and the game that results in a strong advantage that is practically mandatory if you want to do well in the game.
Method did not gain their advantage of having great BoE's by paying Blizzard to give it to them. They gained their advantage by enlisting the aid (often anonymously) of a great number of players to farm the gold needed to pay other players who had obtained (also through playing) good BoE corrupted items.
In principle this is no different to a football (soccer) club spending shit loads of money to recruit the best players, hire the best coaches and managers, get access to the best facilities and experts etc etc etc. It's not paying to win, it's simply paying to get the best team you can, and then the best team winning. Paying-to-win would be like bribery of referees or opposition. Or maybe paying a big bunch of cash and being given a 10-0 advantage before the first kick.