To be fair, supernatural gifts were in the Dungeon Master's Guide too; they didn't originate in Theros. It's just that the ones in Theros and the Dark Gifts in Ravenloft are clearly designed to be equivalent to a feat and designed as a first level feature that everyone can get. PCs in Theros and Ravenloft are supposed to be tougher for reasons pertaining to those settings.
Dark Sun is another such setting that has a history of this: 4e, they were the source of the heroic themes feature that essentially gave your character an additional encounter power (the next one of which you wouldn't get until 3rd level), in earlier editions, you're just assumed to start at 3rd instead of 1st. But a bit later in 4e, Dragon magazine and various products introduced themes for everyone. An additional heroic tier choice was very valued for narrative reasons, and having an extra tool in the arsenal was nice. It just meant that DMs had to rebalance encounters that had been written for earlier in the edition by adding a monster or two - just get the CR up a few levels.
These Gifts are doing the exact same thing to the 5e landscape. Maybe it would be power creep if only some characters get the power, but the books specifically suggest that if someone starts with these, everyone should or should get a bonus feat. Honestly, before Theros, I always granted everyone a bonus feat to begin with, anyway (so that they wouldn't feel beholden to miss out on the first ASI to get a feature that made their character feel right).