esalkin - it would have been easier to reply if you'd identified the post, and so the context, of the quote.
What I think the sentence means is, for example...
You have an empire with no strategic resource to complete troops of a given kind (e.g. no iron for swordsmen, no saltpetre for muskets, no rubber for infantry, etc)
You could trade with the AI to get the resource, though.
What the poster suggests is that, because the AI does not consider how many upgradable obsolete units you may have, you should stockpile those, then trade for the resource, then upgrade them all. That way you maximize the use of the resource, and get the highest value.
An equivalent trick might be with workewrs and coal. If you have got steam power, but no coal, and there is an AI civ with coal, and you don't want to fight a war (for the coal) you want to get the best value for your 20 turns of coal. Therefore, make sure you have the maximum possible number of workers (to build railroads), if possible be a democracy and/or have replaceable parts (to get the workers as fast as possible). Then you will maximize the number of railroads you can build in 20 turns.
All that is being suggested is that you maximize the benefit from a strategic resource trade by adjusting 'other factors'.
I think...