True, but they have also forced the reduction of the key size from 64 bits to 56 bits, making it much more vulnerable to brute force.

So they have hardened DES towards attacks from those with much less money than NSA, while making it weaker against rich organizations like NSA.

They made it resistant to differential attacks yes, but weakened it for their own attack surface

What does that mean?

They made the S-blocks appear less deterministic when sampling their behaviour, but made sure that anyone with prohibitively expensive equipment (say, the NSA) could brute force them in a reasonable time frame.

These are all words (mostly; "S-blocks" is not a thing, you mean "s-boxes"), but they're not coherent in a sentence. I think you're trying to point out that NSA shortened the key length from 64 to 56 bits, which has nothing to do with the DES s-boxes. NSA's interventions with the s-boxes made them more resilient to cryptanalysis, not less.

The thing you're thinking is nefarious is not (unless you're a connoisseur of good bugs and believe that differential cryptanalysis, the most important fundamental technique in block cipher analysis, should have been public earlier). It is in fact the opposite of nefarious.

You're right, I thought that the S-boxes were defined by the number of bits and that the NSA had deliberately shortened them, whilst obfuscating other attack techniques. Thanks for the clarification!