Yes, it is correct. When you create a mailbox enabled Public Folder, it will use the Exchange online default domain name as email address (@keyman .onmicrosoft.com).
If you want to change email address for it, you could Connect to Exchange online with PowerShell, then use the command below to modify email address:
Set-MailPublicFolder -Identity MyPublicFolder -PrimarySmtpAddress ******@contoso.com -EmailAddressPolicyEnabled $false
Make sure the domain name that used has been verified in Office 365.
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