People might tend to take you more seriously if you refrained from resorting to personal insults. Yes, the cited section of the Wikipedia article does start out with the assertion that the algorithm can give the median in linear time. However, without already knowing the answer, that is not at all apparent from the remainder of the description. Your opponents are not "twitterbrains" just because they expect more authoritative citations than a single, rather dense, Wikipedia article without so much as a single hyperlink for further reading.
However, after rereading this thread, the Wikipedia article, several other descriptions of the Median of Medians selection algorithm (several of which explicitly supported the 30%/70% argument), and finally the original article which I was able to track down via Google Scholar, I am forced to agree that this algorithm can be used (in combination with the quickselect algorithm) to find the true median in linear time.
Note that the Median of Medians algorithm, on its own, does in fact return a result between 30% and 70% of the list, not the true median at 50%. Only by using it to select the pivot point for the quickselect algorithm do you get the true median of the list in linear time.