Is Sign-in with Apple ID required if I use an industry-specific third party for authentication?

Question

I'm working on an app that serves a very specific industry niche.

The app mostly uses its own authentication method to sign users in, but since there is a competitor with a very large user base and they offer authentication integrations, it is convenient for many of our users to use their old accounts from there. So we added this integration as an authentication option.

We don't offer any kind of broadly available third-party authentication APIs such as Facebook, Google, OAuth and so on, only the one above.

With that said, I wonder if Apple will require us to implement their sign-in integration.

Apple's Guidelines concede a few exceptions:

Sign in with Apple is not required if:

• Your app exclusively uses your company’s own account setup and sign-in systems.
• Your app is an education, enterprise, or business app that requires the user to sign in with an existing education or enterprise account.
• Your app uses a government or industry-backed citizen identification system or electronic ID to authenticate users.
• Your app is a client for a specific third-party service and users are required to sign in to their mail, social media, or other third-party account directly to access their content.

Considerations:

1. Our users do need to sign in with enterprise accounts (second bullet), but since they may or may not utilize the third-party service I'm not sure it applies.
2. While the last bullet might apply to my case, our users aren't required to use this third-party. It just doesn't make sense to offer authentication methods based on personal accounts, since this app is meant for professional use, and the auth integration is exclusively for the convenience of users coming from a similar product.

Our authentication relies on users' work e-mail domain, which identify their company and is also provided via the third-party above.

I wouldn't expect anyone to create a new account with their own, personal Apple ID on our product since their personal e-mail addresses would be no good to us.

Any guidance or previous experience would be much appreciated!