ask experts: does NIW based 485 need an employment letter?

Just check 485 form, there is a paragraph one employment letter.

"If your adjustment of status application is related to an employment based visa petition (Form I-140), you must submit a letter on the letterhead of the petitioning employer which confirms that the job on which thevisa petition is based is still available to you. Theletter must also state the salary that will be paid."

It mentions you must submit a letter backed up by "petitioning employer". However, NIW doesn't have any petition employer. Does that mean this employment letter is not necessary for NIW 140, but a must for PERM 140?

If this is also necessary for NIW, is the offer letter enough? If not, where should I get the letter? I am a postdoc.

Thanks.