I just slotted the “lifesteal increased by x” to a blank 2h sword (blister, unenchanted) and it added lifesteal to it. the gem is actually giving lifesteal, as described on the enchants google doc.
in my setup it gives me 3 hp per hit.
“Increased Lifesteal - GIVES you lifesteal. It doesn’t increase other sources of lifesteal. Healing increases scales lifesteal.”
but you know that already 