I think it goes further than that, that alien and native lifeforms will probably be toxic to one another. All life on this planet evolved in tandem, and have some of the same basic compounds for life, so there are few toxic interactions between life forms, excluding those evolved for defense (and even then there are exceptions, plenty of plants contain. And even then there are plenty of exceptions. But lifeforms from a completely different evolutionary line may use completely different proteins for basic functions. Even using the same proteins but of the opposite chirality could result in toxic reactions.
I think that even if we found alien life that evolved and developed on a planet much like ours, and used the same amino acids, metabolic processes, and cellular structures, the odds are high that contact with them would result in toxic interactions.
You've got to weigh "we didn't evolve to specifically eat it" against "it didn't specifically evolve to prevent us from eating it". I'm not sure we'll get an answer to which is a bigger factor unless we find some xenobiology and take a bite.
A lot of digestion is pretty elementary, and built upon our stomachs' acid bath -- proteins go in and are unwound and broken down. Most foods that cause problems later down the line contain a protein that both (a) is resistant to the stomach acid, and doesn't break down before hitting the intestines and (b) causes a problem when it hits the intestines.
Barring that - or incorporating elemental poisons like arsenic at levels we can't cope with - I'd personally bet that if you charred a random alien critter over a fire and wolfed it down, it's not crazy that your stomach acids would break down anything that survived the fire and would otherwise harm you, and you'd be able to extract a reasonable mix of sugars, fats and proteins from what you'd ate.
It's not impossible that an entire planet would have some proteins common to all of their lifeforms that both (a) didn't break down easily when cooked or digested and (b) were incredibly lethal to terrestrial life, but there's no ab initio reason to think they would or wouldn't, one way or the other.
The stomach isn't a perfect barrier, there's always something that gets through. If it didn't we wouldn't be able to take drugs orally. Our bodies have an entire subsystem devoted to processing compounds that made it through the main digestive system into the blood stream. The liver is dedicated to metabolizing these substances, in a process literally called "xenobiotic metabolism".
Sure, the odds of a particular compound found in some alien lifeform having a toxic interaction with our bodies is small, but I consider the odds of one of the hundreds of thousands of different types of compounds in that lifeform having a toxic interaction is on the higher side.
Oh, and to be clear, by toxic interaction I don't necessarily mean a poison, per se. It could also have a carcinogenic effect, or if the compound is common enough in the creature and undigestable through the stomach and intestines it could overload the liver, etc.
Yeah, not a perfect barrier, but if it's good enough that I'm not poisoned, do I care? I eat lots of delicious food that doesn't digest optimally.
As for carcinogenic effects, well, depends on what the timelines and cancer-rates you're talking about look like. We eat lots of things on Earth that are mildly carcinogenic over a lifetime, and the fact they are doesn't really stop us.
I think that even if we found alien life that evolved and developed on a planet much like ours, and used the same amino acids, metabolic processes, and cellular structures, the odds are high that contact with them would result in toxic interactions.