It does.... as long as its compatible with the topcoat. Its all schematics however Primer is used for covering problem areas, as sealer is used more for a uniform surface prior to topcoat, Most waterborne paints today are a combination of both a primer sealer are just that and found in the same can, Higher end two stage chems, tend to be more one or the other, Normally thinning the primer down to be the sealer. or two completely different products altogether such as epoxy base (depending on what it is your trying to cover).... Multiple coats of paint, can be worked in the same fashion, although again, that may not seal bleed through after time and may allow for discoloration, and furthermore wont hide sand through but will enhance them in the form of a "Bullseye".
90% of products that you see out on the floor today are compatible. So this
shouldn't be an issue, and this bleed through usually is Tannin leaking though from fresh cut of wood. So unless he is building this thing out of Cedar, Pine, or any other fresh cuts this
shouldn't be an issue. He is building a Cab not a Deck. What you said is true about a Sealer Vs a Primer, but that really only applies in Lacquer based systems, again
not an issue here. In fact almost all of the industrial coatings sold now are incredible primers that need to adhere to slick surfaces. These aren't technically sealers but if you use a product that is meant to stick on slick structural steel I am willing to bet the high solids in that type of primer would never allow for Tannin bleed, but again it's rare to have Tannin bleed in Plywood, and there is 0 chance of that happening in MDF.
And again he isn't using "an old lacquer based system".
This is dejavu we had this talk before. But back then you were insistent that Rattle cans are the way to go. Which is the farthest thing from the truth, unless you like your paint thin with very little solids. In fact every problem you listed above are issues with Rattle cans because the damn things have crap paint in them most times. And the mils of your coating (thickness) has a hard time blocking anything including things like grain and any knots.
So with that being said I understand you want him to use a sealer, but most high quality primers can be built up and fill any imperfections in the wood. Plus they can seal out anything plywood or particle wood has in store, I am trying to make this simple because the spec is super simple. Remember I do this for a living, I work very closely with high production wood shops, to mom and pop shops. I spec out everything from skyscrapers to your neighbors deck, I know what I am talking about because I have seen every issue in the field and every success.
High quality Pimer/Sealer. If the wood you are dealing with has any imperfections you can do multiple coats and sand in between. No problem! Super easy.