I believe that the commenter, Merrill – as opposed to proposing that others should mistrust the entire mainstream media – was instead picking a bone with the NYT’s Nate Cohn and the specific NYT/Siena polls (which have sampling problems as detailed upthread).
Your linked article was an interesting read, especially regarding the “low trust” segment of the populace. I do take issue with the author using current polling numbers as the foundations of their arguments, however.
That said … there does now seem to be some slipping of Biden’s support among young adults and persons of color (though your link seems to admit this is driven by non-likely voter responses). Even the plus-Biden Marist poll Tero just posted talks about this effect. Not prepared to say that Biden’s actually underwater with young voters and BIPOC voters, but I can accept that he’s lost some percentage. How big a percentage – and whether it’s a meaningful amount – I don’t think will be discernable until November.