A couple of colleagues suggested in a discussion that the virus that causes COVID-19 appears to be made by humans, since nature could not have produced such an efficient virus — that spreads so fast and whose patients are contagious quite some time before showing signs of infection.

Since my knowledge of biology is very limited, my only counterargument for such a conspiracy theory was along the following lines:


there is a consensus that the most probable source of the first infection was in an animal market in China.
since that animal market was actually composed of a plethora of animals belonging to various species (mixed with humans), a virus had a bigger chance of evolving a mutation that might infect an individual from another species (a thing that is way less unlikely in the wild since many of those animals do not sit close to each others or next to humans).


Clearly, I have made a little story that might be quite far away from how SARS-CoV-2 infected humans, so I am interested in a scientific arguments to support my cause.

Question: What are the main scientific arguments that can be used to debunk COVID-19 being engineered by humans?

Answers that also include explanations more accessible to laymen are greatly welcomed.

At the moment, there is very little scientific literature about this, but I found two papers that address the problem and are fairly easy to understand. You can find them in the references. Reference 1 is probably the most interesting and is the basis for this answer.
Edit: It is also interesting to read reference 2 on the origin of SARS-CoV-2; the article also addresses some of the conspiracy theories.
As far as I can see, there are a few major points taken up by conspiracy theories.
1. SARS-CoV-2 leaked from a lab in which research on the Bat CoV (RaTG13) was done:
Unlikely, since the viruses share only around 96% sequence homology, which translates into more than 1100 sites where the sequence of SARS-CoV-2 viruses is different from RaTG13. The mutations are distributed throughout the viral genome in a natural evolutionary pattern, making it highly unlikely that SARS-CoV-2 is a direct descendant from RaTG13. For comparison, the original SARS-CoV and the intermediate host palm civet SARS-like CoV from which it originated shared 99.8% sequence homology, showing a much closer relation.
2. The S (spike) protein from bat SARS-CoV cannot be used to enter human cells via the human ACE2 receptor and therefore has been adapted in the lab:
This is untrue, since a 2013 study of a novel bat coronavirus was published showing the ability of the virus to enter cells via the ACE2 receptor. See reference 3 for details.
3. The spike protein of SARS-CoV contains a unique inserted sequence (1378 bp) located in the middle of its spike glycoprotein gene that had no match in other coronaviruses:
As shown in reference 4, the sequence comparison of the SARS-CoV-2 with closely related other coronaviruses shows that this sequence is not unique to the new virus but is already present in older strains. It shows some difference due to inserted mutations.
4. The claim that SARS-CoV-2 contains four insertions from HIV-1:
The paper claiming this has now been retracted due to severe criticism, and additionally a renowned HIV expert published an analysis (reference 5) demonstrating that the HIV-1 claimed insertions are random rather than targeted.
5. The claim that the SARS-CoV-2 virus is completely man-made:
To design such a "weapon grade" virus in the lab, the design would usually start from a known virus backbone and then introduce logical changes (for example, complete genes from other viruses). This cannot be seen in the genome of the virus; rather, you see randomly distributed changes throughout the genome coming from virus evolution and not directed cloning. It is more likely that this virus originates from the recombination of a bat CoV (to which it is closely, but not directly related) and another, not yet known CoV in an intermediate host, like the palm civet for the 2003 CoV.
References:

No credible evidence supporting claims of the laboratory engineering
of SARS-CoV-2
The proximal origin of SARS-CoV-2
Isolation and characterization of a bat SARS-like coronavirus that
uses the ACE2 receptor
Is SARS-CoV-2 originated from laboratory? A rebuttal to the claim of
formation via laboratory recombination
HIV-1 did not contribute to the 2019-nCoV genome

