It's not too controversial that, under
domestic law, the Executive branch can claim precedent from previous OLC opinions to justify the strike as an exercise of POTUS' Article II powers. It's understood that nearly all Executive branch lawyers (and Johnson was the DoD General Counsel at one point) argue for an expansive interpretation of the Executive's Article II powers. It's also understood that Congress has consistently abdicated its position on these matters for decades.
Johnson's position has nothing to do with his employment by Lockheed, but because his office authored some of these OLC opinions which the Executive branch is or will claim as precedent. I'm not discounting some self-serving aspect, but if you asked Happy Retired Grandfather Jeh Johnson vice Lockheed Board Member Jeh Johnson, you'd get the same answer.
I think this interpretation is wrong, but it's a domestic/constitutional question... and what's completely missing in most analyses is any discussion of international law, which is just as binding as domestic law. That discussion should assess the imminence of the attack and how that relates to the inherent right of self-defense under the U.N. Charter.
Of course, the Administration's justification, and presumably the analysis, is classified. But Pompeo has already told several versions of the story, and that contradicts DoD press releases.
The Constitution is NOT the only "law of the land" -- treaties are too, like the Charter.
(See more by Oona Hathaway:
https://www.theatlantic.com/ideas/archive/2020/01/soleimani-strike-law/604417/.)
Incidentally, I'm 100% within your camp on this one. Just for different reasons.