chemtrails form much closer to the engine because there are actual soot products (chemicals) being released and that is visible right away
the condensation process of contrails takes just a little extra time for the engine heat to interact with the moisture already in the air to cause the condesation
thus the observable gap and also rather quick dissipation
but the real tell is cargo capacity of a learjet
it just would not make a good chemtrail platform because of weight limitations
the tanks are heavy af when full
that is why they use the big planes
it just would be too inefficient to try and do it with smaller jets