You can see from this picture that he used very thick hinges,box tubing and thicker plate welded to thin mazda sheetmetal
That over time is going to get pressure cracks, leading to door not closing properly possibly. By the time that happens the truck will most likely be rebuilt or in scrapyard.
Is it properly done? somewhat. Those plating inside to original mazda metal isn't much. Doesn't matter if it's 1 inch thick gussets, your still welding to thin sheetmetal. You need to spread the load over a greater distance when you weld to sheetmetal.
What should have been done is 4-5-6 holes drilled on upper side of that square cutout and lower side of that cutout. Then take a 1/8 plate with 3-4 holes drilled over section where the hinge will be located. You then insert into that cutout and plug weld on upper/lower portion and then plug weld the 1/8 plate to the hinge/box tubing. Obviously allowing access to hinge bolt.