I’m sorry but this does not make any sense.
To talk about “those outside the garden” is to talk about people who are located somewhere when some event took place. It’s in the very label. They are defined by their location in space and time, not their genealogical line of descent. Presumably the distinction is important because they are not those who were in the garden and doing the sinning.
Hence those that were in the garden and sinning, would be unjustly transmitting their sins(assuming this is how it works) to those outside the garden, when they ventured out and had children with them. Those children would still be outside the garden, not having actually participated in perpetrating the original sins. Thus to say they now share in the blame, or guilt, or responsibility for, or what you might call it, of having sinned, is to my eyes completely unjust. What is it they, the people outside the garden, actually did that now makes them sinners?