You have to put something between the concrete and the engineered flooring. Whether this is a thermal barrier or plywood depends on what type of engineered flooring you put in and how much moisture the floor has.
All concrete floors have moisture. This might be next to nothing or a considerable amount, even in an unnoticeable way. To make matters worse the engineered flooring you are getting might be pretty closed system or rather breathable it is really hard to tell. Also with an on-grade install the moisture levels of the concrete can vary by season or year. Heavy rains for a month might cause a slight issue after 5 years of no moisture at all.
I have two pieces of advice:
If putting engineered on an on-grade slab make sure the backing of the flooring is a real wood - in a hard plywood form. No mdf/mush/particle-board type of back - period. They will warp on you. I mention this because most stuff that is a decent price at most local stores will be made out of crap backing (don't care if it has padding or not). These products are only for the simplest of installs - on commercial type concrete or in a residential above grade. An easy test is take samples and put them in a bucket of water for a few hours, let them dry and see how they look. The other option is getting pure vinyl click lock which is getting pretty damn close to engineered in the way it looks. You have to look pretty close to tell its not a wood product for the good ones - which can also mimic tile.
Follow the manufacturers instructions - call them if you need to. Not the store's instructions, the manufacturer's. If they tell you, you can install it directly on the floor but they suggest a 1/2" plywood - do the plywood. I personally haven't heard of many manufacturers of engineered flooring that will OK direct concrete install that is on-grade but who knows. Got to hit their website and call them. 20 mins of research can save you time and headache.