The way I see it it is one of three possibilities:
1 - Sonic is offering AT&T, so they are not planning to run their own in that area
2 - Sonic is offering AT&T, so they de-prioritize those areas and focus on those who do not have any fiber
3 - The two are completely independent of each other, Sonic is offering AT&T where it can, and stringing own fiber where they can
Anyone know which it is?