Need help with SHOULD

The following paragraph has been taken from the American Heritage Book of English.

http://www.bartleby.com/64/C001/057.html (This is the link to the page.)

should versus would. Just as they ignore the traditional rules governing the use of shall and will, Americans largely ignore the traditional rules governing the use of should and would. The two verbs are not always interchangeable, however. You can use either should and would in the first person to express the future from the point of view of the past, but keep in mind that should sounds more formal than would: He swore that I should (or would) pay for the remark. The same principle applies to the verb in sentences that express a future condition: If I had known that, I would (or more formally, should) have answered differently. In the second and third persons, however, you only use would: She assured us that she would (not should) return. If he had known that, he would (not should) have answered differently.

According to this paragraph, the sentence which is highighted in red is refering to a future condition. But according to rules, it's an "if clause type III" which only refers to the past.

For references to the IF Types you can visit the following links.

• 
  
  http://www.ego4u.com/en/cram-up/grammar/conditional-sentences
  


• 
  
  http://www.ego4u.com/en/cram-up/grammar/conditional-sentences/type-3
  

Please can someone tell me wheather it's a mistake by the author or it actually has something to do with the future.

GB