We don't have one single definition of life, but we do have _definitions_. There are typically a few properties we think living things share, to rule out things that are not living from those that are living.

For instance, crystals reproduce themselves, but we do not consider them living. So we decide that livings things use energy, which rules out crystals as being alive.

Fire reproduces itself, and it uses energy, but we don't consider it to be living, so we say that living systems preserve information http://scienceline.ucsb.edu/getkey.php?key=360 .

Viruses reproduce themselves-- in fact, using the same information-containing molecules that living cells do, DNA and RNA, but they do not use energy (instead they hijack the metabolic machinery of living cells). So we do not consider viruses alive.

So in various definitions of life, you will find criteria such as "uses energy" (has a metabolic process) and "transmits information" (in the form of DNA).

That we have defined such criteria does not mean we arbitrarily made up such distinctions. Human beings differentiated between mechanical and electromagnetic energy, or had the concept of "species", long before we had strict scientific definitions of these things. It is the same with living and non-living phenomena. The only difference is that we arrived at a chemo-physical definition of living much later than other phenomena.

Saying that life is not objectively real is nonsense. That's like saying there's no objective distinction between mechanical and radioactive energy. They're objectively different phenomena, in theory and in measurement. The ultimate conclusion of this line of reasoning is that any distinction between any thing or phenomenon is not objectively real, just a fancy of the mind (which you will find agreement on with Hindu and certain Buddhist philosophers).

Or perhaps you are actually claiming that rocks are alive? Or there really are no differences between a rock and a lichen? If there is no objective, "real" difference between living and non-living things, then a quartz crystal is actually alive (and perhaps conscious) like new-agers claim?

In the case of your aliens, I would argue they are intelligent, but not alive-- sort of like a robot or AI machine. Those aliens are like a side effect of that chemical process. (Unless they in turn re-created the chemical situation that they originate from-- but then of course, they would be reproducing, and thus, living things).

Your aliens aren't that different from if we sent drone robots that we created to aliens. Those drone robots aren't living, though they may be intelligent, and interact intelligently with the aliens. But they don't reproduce; they are a side effect of another living chemical process-- namely, human beings. Unless, of course, those drone robots managed to re-create themselves, or created humans which in turn replicated drone robots.

In the mechanical wombs thought experiment, we are still alive, because we are reproducing. Your mechanical womb is basically an artificial egg. A chicken is just an egg's way of making another egg, after all.

Would you consider a test-tube baby not to be alive?

A multicellular organism is epiphenomical of the cell, and the cell is epiphenomenal of the replicating DNA molecule. Life doesn't start with complex, conscious multicellular organisms like ourselves; it starts with simple, replicating molecules-- namely, DNA.