You're using Thunderbird hence it is "managing" your account.

I doubt there is anything anyone can do to stop Google from "managing" the accounts it provides to its users.

What is it that you think is going to happen?

When I click the 'i' button to see what is involved, I read it will "View, manage and permanently delete mail, Create, update and delete labels, Compose and send new email" Maybe it is the way this is worded, but I feel I wish to, for example, "Ctreate new email" I don't know what the labels are for, but if I want them, I want to "Create and delete" them. I set up a gmail account on another PC in Thunderbir under Windows 7, and am still using it having updated to Windows 10. I have not been asked to allow Mozilla / Gmail to 'Manage my accounts' on this PC.

Labels are gmail's way of managing email messages. The classic approach used by email clients is to mimic the folders you find in your computer's file system. So in a regular email account, if you copy a message to two different folders, you have two distinct copies.

Gmail holds one copy, but tags each one to indicate in what contexts it will appear. This means that if a message is presented in two different folders, there is only one instance of that message, but it has tags for the two possible places it might appear to be in. This is marginally more efficient in terms of disk space.

To all intents and purposes, both Thunderbird and Gmail's own mail software present messages as if they were actually in folders. If you ask to see "Inbox", Thunderbrd presents the Inbox folder and its contents. Gmail presents just those messages whose tags say they are appropriate to be shown as if in the Inbox. Deleting and creating tags is pretty much invisible, and is just the gmail mechanism for determining what it shows to you when you select a particular view or folder. Labels are automatically created and deleted as you move messages from folder to folder in Thunderbird.

https://support.mozilla.org/en-US/kb/thunderbird-and-gmail

The thing you're agreeing to is very like a contract; it's a formal statement of the interaction between the email client and the email provider. As such it is unavoidably a bit formal in its style, and like legal talk, has to use specialised terminology to avoid ambiguity. So it has to mention labels since they are the actual implementation. But to use gmail with Thunderbird you don't really need to know the details of gmail's tagging mechanism. Just as you don't need to understand internal combustion engines to drive a car.

BTW, don't you think "Compose and send new email" equates to "Create new email"?