I don't mean that they are paying for someone else to host the files; what I'm saying is that the files are taking up storage space that could be used for something else. So take the following as an example:
Current size of Apple's website (including obsolete software) = a.
Size of obsolete software = s.
Now let's suppose that Apple need to make their website bigger. Let's call the new size b.
The amount by which the website has grown in size, g, is b - a.
If Apple were to keep the obsolete software on the website, they would now need to purchase g units of extra storage (whether that be off-site hosting, cloud storage, or hard disks - it's irrelevant, they would still need to get extra storage).
If Apple instead took the obsolete software off of the website, they would only need to purchase g - s units of extra storage.
Clearly, no matter what their hosting arrangements are, they are saving money.