Thank you for those quoted examples, which help to highlight the issue in hand:
1. This sentence "He thinks he's a Sheol of a farmer; thinks he's old Grayback from Wayback" does not present problems to learners if "from way back (or from Wayback here) is an adjectival modifying "Grayback." The same account goes for "he's an artist from way back!" "Librarians are master networkers from way back," and "It's a T-shirt from way back." There is no issue of tense incompatibility as the "from way back" functions adjectivally.
2. I dined and wined and toasted him from way back.
This one, cast in the past tense, accords with most learner's understanding if "from way back" indicates some time in the past. The other examples follow this pattern.
However, sentence (b) feels incongruous as it is cast in the present tense and "from way back" is not an adjectival there. In other words, it does not belong to either of the above groups of examples.
b. John and I know each other from way back.