Well, no. A +6 would change the success rate from 1/8 to 3/8 - a tripling of success rates due to the bonus. Something that bumps almost half of your failures into successes is a major change.
Something that bumps one failure out of eight into a success isn't going to have a whole lot of impact on the game, especially when a single character is unlikely to make more than about 8 skill checks in a given session. A +2 attack does have a larger impact simply because the player makes so many more attack rolls than skill checks.
That's why I talk about tracking the impact. Because we don't really know how often a character is making a skill check in a given session and how often that that skill check succeeds because of a bonus d4. You'd need to track over several sessions, across a number of characters, but, I would hazard a guess that the impact of Guidance is far, far from game breaking.
I think this is where our disagreement stems. In my game I know for a fact that 8 skill checks is low. Most of my games have many more skill checks than attack rolls, unless it’s a combat heavy session