There is always a chance that I am wrong here… But I do not recall being able to turn off the welcome email being send out when a user is created as active.
The reason why I think an email is always sent: the email sent to the user contains a unique (and expiring) link for them to set their password. Otherwise you needed further actions from the Admin (to set/communicate the new password, or trigger a password reset which would generate an email anyway)
Things are changing with the move to Autodesk Identity, but there is still the need to communicate to the user a unique link if we cannot find that this email is already associated to one. The invitation process will pre-reserve a user with that email.