Midlifeflyer is correct; ditch driver is alluding to something covered in my previous post. That instruction is within the duties of the compan instructor, who is providing instruction within his designated limitations. That company instructor may not teach an outside pilot how to fly gliders, or teach a company mechanic, lineman, or someone else, to fly in the simulator at night...that's outside the program limitations. Likewise, if his duties involve the EMB-120, he can't go to another company simulator, rent it, and teach the ERJ-145 at night, unless he's a certificated flight instructor and typed in that airplane, as it's not his company, not his company program, etc.
The concept of using the ATP, which is the subject of this thread, to instruct beyond that allowed in instructing other pilots in air transport service, is occasionally stretched too far. Likewise, when one is a company instructor, that doesn't make that person a flight instructor beyond the limitations of the company training program. Obtaining a type or an ATP certificate (same ride) is the same as taking a 135 or 121 flight check...same standards, same ride. When the applicant is endorsed, he's being endorsed as completing an FAA approved training program; it's really the program that he's being endorsed for, not the evaluation and instruction capabilities and insight of the company instructor. The company instructor, whatever his or her certification, is merely acting as a signatory representative for the approved training program.
As such, he or she may provide no additional instruction, or endorsement, beyond the program itself. No addition types for which the program is not established, no additional certificates or ratings not applicable to the program, etc.
It was for this purpose that the FAA adjusted and reissued the definition of "authorized instructor" in Part 61.1(b)(2)(iii) of the regulation:
(b) For the purpose of this part:
(2) Authorized instructor means -
(i) A person who holds a valid ground instructor certificate issued under part 61 or part 143 of this chapter when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
(ii) A person who holds a current flight instructor certificate issued under part 61 of this chapter when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
(iii) A person authorized by the Administrator to provide ground training or flight training under SFAR No. 58, or part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.