The argument is that naively looking at the price tags of domestic and foreign weapon systems doesn't tell you the true costs to the country. A dollar given to a foreign country costs much more than a dollar spent inside your own economy.
This is hardly even a novel or controversial point. Any defense spending expert will tell you this. Even that trendy Perun guy that reddit loves has made this precise point.
You are being deliberately obtuse. Investing in domestic production isn't an infinite money hack, that isn't the argument. The argument is that you cannot make a apples-to-apples dollars-to-dollar price comparison to see how much it will cost your country to outsource production of hardware. With the benifit of hindsight, we know that Diefenbaker's decision spelled the death of Canada's aerospace industry. Their engineers moved to America or left their careers behind. The cost to Canada of buying American jets was considerably more than the sticker price of those jets.
If this were true, Argentina would be super rich and Singapore would be super poor.
But it is not because just “keeping a dollar in your country” chasing less productive work/goods causes productivity and competitive problems the more it happens and longer it goes on.
There are plenty of defense experts who’ll say the opposite of what you say they will (though they’re open to exceptions for security reasons).
It is better to buy cheaper/better stuff abroad and have others do the same for your competitive stuff.