"worth" can have two meanings in this context. $100 from 1917 can be worth exactly $100 today. Or it can be worth what you can buy with it.

Some folks will see a $100 bill from the era and see an old $100 bill. Some folks will imagine what that $100 took to save back then, and what it bought.

FWIW my brain automatically went with "the goods that can be bought with $100" - such as what I could buy in a grocery store today with $100 would be about what I could buy with $3 back then.

I never considered the other reading until this thread. It was obvious to me the author meant "you can buy 97% less stuff today with the same $100".

I don't understand how you mean. They say $100 back then, in what meaning is a $100 bill back then worth the same as having $3 today?

I think it's used to convey that the buying power has been reduced. If you have a $100 basket of goods (as measured in 1914 dollars), $100 in 1914 allows you to buy 1 basket of goods. Due to the devaluation, today spending $100 would only give you a $3.05 basket of goods (as measured in 1914 dollars).

It's a bit of an odd comparison since you're using two different units for dollars to compare the basket vs purchasing dollar. The clearer way to say it is that today's $100 basket of goods is equivalent to $3.95 basket of goods of 1914.