True as long as the board isn't bowed. Early on, I had a bowed poplar board and a shiny new Jointer/planer. I got the board flat but by the time I was done the center was 3/4" thick the ends were 3/8" thick. Oops. Lesson learned and didn't cost much at all.
As far as jointer bed length and max board length, a rule of thumb I've seen is the board no more than 1.5 X to 2 X bed length. My Jet JJP has a 55" overall jointer bed length. Using the rule of thumb my max board length would be 82" (1.5X) to 110" (2X). For me, 7' is plenty without auxiliary support. I don't do large projects so that works for me. If I routinely did dining tables, entry doors and beds I'd probably think differently.