The paper specifically measured respirable particles, which are particles small enough to get into the lungs and bloodstream and cause health problems due to sort of mechanical/local effects, which includes chronic bronchitis and cancer. Of course there are many, many more substances in second-hand smoke, many of which are carcinogens. Many of those substances, such as carbon monoxide, can also be fairly easily measured to determine whether they are sufficient to pose a health risk. Now, because second-hand smoke tends to fail the respirable particles test, the rest don't really matter. But conversely, just because second-hand smoke passes some of the tests, doesn't mean that it is safe. Even if it passed the tests for CO, nicotine, respirable particles, formaldehyde, benzene, etc. you still have to deal with the problem that there are many other carcinogens present - some of which we've identified and some where we have a decent idea about safety thresholds, but some that we haven't. We can't simply use ignorance to set safety levels ("what you don't know won't hurt you") because we already know that cigarette smoke causes cancer. This is, it's not a case of assuming everything is harmful until proven otherwise, but of dealing with something which is proven harmful. We can try to use epidemiological studies to get some idea about the dose which is associated with harm. And even if you want to argue about whether we are confident that the studies are adequate to prove a specific level of harm, it is unarguable that they do not prove that typical doses are safe.
However, most of this is moot, since indoor smoking usually fails at least one of the more straightforward standards anyway.
Well, if you are truly ignorant of this process, you could spend some time reviewing the reports from various boards charged with health and safety. I gave a link earlier to the Report on Carcinogens. If you click on any of the substances, you can read through the summary which answers those questions (and much more) for each substance, as well as providing references to the available research. You can usually find reports from similar departments in other countries. The results are much the same, since the same information will be used.