Is my Theory about my Foster dog Correct?

I've noticed a pattern surrounding our foster dogs dog selectivity, and was curious if anyone has any experience with something similar and/or could confirm this (:

Background: My husband and I have had our foster for about five months. She's amazing with people and truly has never met a stranger. I would consider her dog selective. She's great with our two resident dogs (they play, nap together, and coexist well), but she's reactive toward unfamiliar dogs both on and off leash. She'll bark, growl, lunge, and raise her hackles. We've made a lot of progress with training, but the reactivity is still there.

We don't know anything about her past because she was found abandoned in a park. Based on her anatomy, we suspect she may have been used for breeding and spent much of her life kenneled, though that's only speculation.

The pattern: Every dog she's successfully met has been with someone she already knows and trusts. One of my friends, whom she absolutely loves, has brought over several dogs of different ages, sizes, and temperaments. She's had no reactivity toward any of them, is able to walk around and lounge with them, and has even played off leash with some. The same thing happened when she met a dog belonging to the founder of the rescue, another person she knows well.

On the other hand, if the dog is being handled by a "stranger", she reacts immediately. This included our neighbors.

My theory: Since she genuinely loves people, I don't think the issue is with strangers themselves. I wonder if she lacks confidence around unfamiliar dogs and uses the handler as a "trust filter." If she trusts the person holding the leash, she assumes their dog is safe too. I also wonder if she's simply had very little healthy socialization with other dogs. She has an unusual play style, seems to miss some canine social cues, and has escalated appropriate corrections from our resident dogs into fights- which makes me think she may never have learned normal dog communication.

Has anyone worked with a dog like this before? Is there a behavioral explanation for why she seems to trust dogs based on who they're with, or am I reading too much into it?

Also, if this theory has any merit, how would you go about introducing her to a potential adopter's resident dog? Thank you in advance, im curious to know what everyone thinks (: