What happens in Howlying Abyss does actually happen in Summoner's Rift, to a certain extent, which is why it's valuable to consider.
It happens a bit, and what is important to note is that Summoner's Rift has many, many, many counters to this form of gameplay that Howling Abyss doesn't allow for. Examining a form of gameplay in an environment that is designed to remove all counters to that gameplay while also enhancing the power that gameplay brings doesn't give you good information.
Oppressive strengths in a specific area probably has a lot to do with that if you ask me.
I'd be more inclined to believe this if their nerfs were targeted at the strengths that Howling Abyss thrives on. Movement speed and her Q power cord damage are tangental to her strengths there. What they cite as being oppressive about Sona is her laning against anyone who doesn't counter her.
Here's why Sona is queen there: she has better sustained healing than anyone else, and she has a lot of power in pure 5v5 situations. These matter more in Howling Abyss because you have no options for healing during a siege and because you will always be in a 5v5. Neither of these are true on Summoner's Rift. Drawing any sort of conclusion about summoner's rift based on situations that don't exist there is not valid.
Being the indisputable queen of a map that has a decent amount of similarities to Rift is a good place to start looking for oppressive strengths.
I don't agree. It's not similar enough to warrant useful information. In order to examine whether or not something is oppressive, you have to examine it in an environment that actually has the full options for counterplay avalivable. After all, something is oppressive when an opponent has no options for how to play against it. Howling Abyss eliminates the vast majority of counterplay options to the prolonged siege gameplay. Things can easily be oppressive there that are completely If Riot actually did this, and used Howling Abyss as a nerf motivation, then it was a mistake on their part and they need to spend time seriously reevaluating their methods.
I'm sorry if I got across the point Sona's state of balance in Rift can be taken from Abyss, that wasn't really the point I was trying to get across.
Focusing on Sona's strengths is a useful tool for dealing with that question.
Why riot nerfs her.
There is a problem in what you are saying here. You are saying Riot used this data to determine if she had an oppressive play pattern, but then they chose to make a pure balance decision by nerfing her instead of changing her play patterns. They also nerfed aspects generally unrelated to her Howling Abyss success rates, and cited reasons unrelated to it. Given their actions, I don't think your conclusion is valid. If it is, then Riot not only made a mistake in using Howling Abyss to draw conclusions about Summoner's Rift balance, but they also drew conclusions that did not follow from what could be observed.