TL;DR: DeJoy's changes appear to have made USPS slightly slower (0.1-0.5 days) in some places, but much less than the delta caused by a holiday like July 4th (see note buried at the very end).
I think the median and mode delivery times would be helpful numbers rather than just mean.
* Prescription refills
* Mail to/from overseas soldiers
* Bills and bill payments
* Regular letters
Those other categories could be much worse, or could be better than Shippos' data here. Most of the reporting out there claims that other categories (like prescriptions) are severely degraded.
You say "slightly slower". The metrics behind the delays are much longer and your numbers only seem to include time in transit.
Second reason your summary is inaccurate: DeJoy's changes are rolled out in small pockets of the country thus far. The article seems to be indiscriminate from where those changes took place. The averages here are hence better than from those places where the changes have done the most damage.
Where are you getting this causation from?