If I understand you correctly, your client doesn't have any LTCG, only ordinary dividends (and perhaps qualified dividends).  If so, you seem to have overlooked the instructions for Line 17, where taxpayers are directed to skip Line 18 through 21, and go directly to Line 22.  That's why neither box on Line 20 is checked.  If your client has qualified dividends, PTO will generate a Capital Gain Tax Worksheet and calculate the tax correctly without impacting the investment interest allowed on Sch A.

In other words, PTO is correct in how the return is prepared, under the circumstances you described.