Oh come on. You know what he means.
And you could argue humans were designed by evolution. I don't think the OP meant some alien in a garage sat down and just drew up what humans should be.
The problem with calling us "designed" is that it makes it sound like we are all built to some universal specification. This is not true, but the parent post acts as if it is. Whatever evolution does (call it design if you must), it does not act particularly strongly to get all humans the same potential range of motion.
Edit: to illustrate why I think it's so wrong to make such a statment, let's consider one of its implications. If we all have the same design range of motion and all differences are caused my muscle imbalances, then when I see someone do something I can't do, I have to conclude that the difference is a muscle imbalance. I wouldn't be surprised if this was the difference most of the time, but I think it's clearly false to say that's the correct conclusion all of the time. So I would sometimes be wrongly concluding that muscle balances are my problem and might wrongly attempt motions that I'm incapable of or waste time on exercises that can't help.
On top of the skeleton we add muscles and tissues and everything else, with the muscles doing the vast majority of the supporting work for the skeleton, i.e., for the most part, the muscles (and tendons) dictate the effective range of motion, i.e., what the skeleton is currently possible of doing. But muscles can be stretched and lengthened, or they can be neglected and become stiff like an old rubber band (e.g., from sitting in the same position for years on end). Stiff muscles restrict range of motion, preventing the skeleton (which, without the muscles, would be fully capable of achieving its design range of motion) from achieving the full range of motion.
Are we all built to some universal specification? That depends on how you define "built" and how you define "universal". Our cells "build us" and our DNA defines a specification. Humans do follow a general standard: between 4.5-6.5 feet tall, two legs, two arms, one head, bipedal motion, etc. There are no 3-foot, 8-arm humans (like an octopus) and no humans with a 8-foot nose (like an elephant).
There are plenty of factors that can affect what range of motion each of us will be able to achieve, including injuries, age, and lifestyle (you're not going to become a world-class runner sitting at a computer for 12 hours a day 7 days a week), but if you're a relatively healthy 20-50 year old, and you put in the time required, you can go from being extremely stiff and inflexible to doing splits and backflips.
The link I provided in my first post did exactly that and came to the opposite conclusion - that there are differences in range of motion due to skeletal configuration that are significant in at least some activities. It wouldn't be the first time that a fitness blogger was wrong, but the images are pretty compelling.
I'm making a much weaker claim than you are arguing against. I'm making the claim that there are differences between people, and that these differences matter to our range of motion. Pointing out that there are universal generalities between people is not relevant to the discussion. On the other hand, you are making an incredibly strong claim: that there are no noticeable differences. You have provided no support for your much stronger claim.
It's called DNA. We all share most of the same blueprint, and therefore, the same design.
You're problem with "designed" may be that it implies a "designer", i.e. a God. Whether you choose to believe such a concept makes no difference to this subject. There is no need to approve or deny such a thing in this conversation. The end result is the same.
Because we each have very similar DNA, we each have very similar bodies. There are clear differences with each individual, some of these differences come from differences in an individual's DNA, others come from the course of the individual's development. A person's range of motion is heavily dependent on the latter, especially when certain muscles are underutilized.