This isn’t an information disclosure vulnerability that would allow someone to gain knowledge of new Apple IDs. It also doesn’t affect first-party applications.
I can’t provide an explanation of the behavior you observed without more information, but I can reasonably conclude that the vulnerability here wasn’t the cause.
I find it hard to believe that signing up for an Apple ID caused the start of the phishing emails unless the email account or computer has been compromised. This is not normal when signing up for an Apple ID.
My guess would be that it was just a lucky guess/bot sending to a lot of addresses. I’ve had email addresses get spam before without using them anywhere.
I can’t provide an explanation of the behavior you observed without more information, but I can reasonably conclude that the vulnerability here wasn’t the cause.