In the blog post, they Clearly said you cannot unlock XP boster by buying tiers, only unlock cosmetic.
Here are the extract from the blog:
The Season Pass awards free Season Boosts. Boosts accelerate players’ progress for the duration of the Season. For example, a Season Boost might accelerate XP earned to make leveling multiple characters within a season faster. Because they affect gameplay, Season Boosts are free rewards for all players. We want to be clear that players can’t unlock Season Boosts more quickly through purchases–there is no way to unlock more boosts, or boosts at a faster pace, by spending money.
Players can purchase Tiers-but they won’t speed up getting Season Boosts. Players can’t upgrade Season Boosts just by purchasing Tiers, because they’ll also have to earn level milestones to apply them. All other Tier rewards can be unlocked instantly by purchasing Tiers. In other words, there’s no way to shortcut getting Season Boosts by buying Tiers; they must be earned.
Its another clockbait youtuber cashing in on the D4 /Bizzard bashing train .