robtillaart:
What I think is meant in the article is that you should not put the LIPO and the wall wart in parallel to power the Arduino.
That would be bad, no doubt.
There often seems to be concern about powering the UNO in particular, from Vcc or Vin ("barrel jack") whilst connected to USB. Given that the regulated voltages reasonably match - and particularly that there are a polyswitch and a cable of as it turns out, significant resistance between, this would seem most unlikely to cause any problems.
As best as I can determined from the datasheets, applying a voltage which may slightly exceed 5V to the output of the regulator will not cause any problems, apart from a small current loss (approximating its quiescent current draw) through its internal reference.