Yes, in a sense that's true. If you notice the RiffRaff adapters extend into the port with an O-ring around the extension. https://www.riffraffdiesel.com/front-abs-grease-able-block-off-fitting-99-16/Dude, it's literally a little piece of metal with 2 holes in it- might take a half hour to whip up in the shop....
I guess a flat piece of metal drilled and tapped would work, too. I think I'd want it to extend into the ABS port much like the ABS sensor does.