OK, I might know what's happening.
Your screenshots are for the destination, not the source.
So your column comes in as string(100), goes through the SSIS pipeline unhindered and then out to the target table, at which point the truncation happens.
If you want to generate an error, open the Advanced Editor on your data flow source component, and select the 'Output Columns' node, under 'Input and Output Properties' and change the column width there.
Next time you run it, a truncation error should occur.
If the answer to your question can be found with a brief Google search, please perform the search yourself, rather than expecting one of the SSC members to do it for you.