this shouldnt be possible because,
a. marksmen and juggernauts are both different types of carries.
b. marksmen have auto attack range as an offensive+defensive tool and are supposed to work by kiting their opponents. they often even have tools inbuilt in their kit to help keep their distance.
juggernauts are slow moving melee carries who are supposed to work only in direct melee trades. as much as marksmen should be reliant on staying out of close range of their opponents to function properly, juggernauts need to get into melee range and stick.
the described scenario is the largest possible fail scenario for an adc and the best case scenario for a juggernaut. as long as the gap in gold is not exteme, under no circumstances should a marksman EVER be able to take and win that fight. if they can, then something is wrong.
in this case imo its because of two reasons. for one, lifesteal and defensive mechanics for marksmen are massively overtuned. the other reason is, that this is the perfect example why garen cant function as a carry as long as he doesnt get access to some form of in combat sustain himself.
losing to marksmen with lifesteal is the extreme case. but he also loses extended trades to any fighter with in combat sustain or lifesteal built because of it, even when hes ahead in gold.
hes never going to be more than "try to burst your enemy in one ability rotation or get drain tanked and die" until he gets some option, anything really when it comes to in combat sustain.