Refer to this picture:
The job offer is what is the "updated letter" that they are requesting when they asked for additional documents. That just outlines the job description, pay and etc. The offer of employment is the IMM 5983 form. Basically job offer is a personalized letter from the employer to the employee whereas the offer of employment is an official form that needs to be filled.
So when people got their additional documents request from IRCC. You are supposed to provide both a job offer letter AND offer of employment form.
I hope that can clarify things a bit more.
Also, a job offer letter was not originally required when the inital application was sent otherwise the application would have been sent back as incomplete or they would have asked for it before processing things this far.
Side note: Honestly reading all the different ways IRCC have used offer of employment and job offer is confusing for sure. But I think as long as they aren't returning your application when you initially apply and they aren't asking for more documents after the additional documents request, you can be sure that you are submitting things correctly. From what I believe, they have no merit to deny your application on the basis of insufficient documents, they HAVE to ask you for more documents and only when you are unable to provide them will you be rejected.