Under the DMCA safe harbor provisions, the platform is protected from liability if they follow specific procedures, but may be liable for contributory infringement if they do not. The crux of the procedural dance is their receipt of official notices and counter notices, and their responses. The trigger is a sworn statement by the copyright holder: in the scenario that you describe, there is no such take-down notice. A take-down notice is directed at a specific infringement (delivered to Youtube), and the required remedy is that the content host must remove the copied material. This was done in the particular case, so there is no further issue.
The copyright owner would have to separately serve notice on MeTube or FreeTube for distributing the same work. SE or similar platform is not even close to the liability ballpark until it gets served with notice. YouTube, on the other hand, could be liable if they restored the content after taking it down (this is covered via the counter-claim and put-back provisions). The copyright owner's whack-a-mole department has to dilligently find all of those infringers.
"Infringing" boils down to copying, not to "referring to", and in the cases you are asking about, there is no copying of the content – there is a reference (link) to allegedly infringing material. The case Intellectual Reserve, Inc. v. Utah Lighthouse Ministry, Inc., 75 F. Supp. 2d 1290 gives indications of what actions other than actual copying might be deemed "contributory infringement". The court points out that browsing anything requires copying – "in making a copy, even a temporary one, the person who browsed infringes the copyright" –
although if the person browsing "was not aware and had no reason to believe that his or her acts constituted an infringement of copyright, the court in its discretion may reduce the award of statutory damages". So yes, there is always potential for end-users to be sued for infringement, and usually an escape hatch for innocent infringers.
The other question raised in this case, about the platform's liabilities, is somewhat mooted by the apparent age of the would-be infringement which sees to predate DMCA. But the case does include an element of infringement-by-link, and the court found in that instance that "defendants have actively encouraged the infringement of plaintiff's copyright", citing various facts where the host overtly directed users to infringing sites, for example "Please mirror these files . . . . It will be a LOT quicker for you to download the compressed version . . . Needless to say, we need a LOT of mirror sites, as absolutely soon as possible". This is markedly different from what happens on SE. Inline linking is generally considered safe (the burden of stopping copyright infringement is on the actual host). There is no encouragement of infringement, only the potential.