I'm dumb and apparently don't know how to read...
Alright, after looking over the instructions and double checking my steps I realized I had edited amavisd.conf rather than amavisd.conf.in. After editing the correct file my dspam_stats -H results look like this:
TS True Positives: 0
TI True Negatives: 10
IM False Positives: 0
SM False Negatives: 0
SC Spam Corpusfed: 2398
IC Innocent Corpusfed: 5551
TL Training Left: 0
SR Spam Catch Rate: 100.00%
IR Innocent Catch Rate: 100.00%
OR Overall Rate/Accuracy: 100.00%
So based on the fact that there are 10 True Negatives I'd assume it's now working correctly.