Chemical synthesis via 3D printing?
The authors use a 3D printer to generate the labware, and to place the reagents in its inlets.

So we're not printing standard ink on paper but using a the print head to exactly place very amounts of monomers at certain positions. These monomers cure (polymerize) upon heat or irradiation to a solid resin. By this, small reactors of arbitrary design (think in 3-neck reactors) with a volume of ~ 10 mL may be printed layer by layer. 

Apparently, it is even possible to create reactors with windows to allow for later reaction monitoring or print and assemble electrodes for spectroelectrochemistry.


  How can one "print reagents"? What is meant by this?


This is all about precise positioning of the printhead! Thus same printhead used for the ink can be used to fill the inlets (think in dropping funnels of normal labware) of the reactor with componends whose reaction you want to monitor. Instead of dropping, material transfer is achieved by applying a vacuum to the reactor through a self-healing hole.


  Most synthetic reactions require hours of refluxing and whatnot [...]


Well, a lot of reactions do proceed quite well at room temperature, think in


Diels-Alder reactions of furanes and maleimides
electrochemically triggered reactions
photoinduced electron transfer reactions


just to mention a few.


  [...] not to mention isolation/purification afterwards. How can all of these techniques be performed with a printer?


Oh, you got that one slightly wrong. The "printer" is just used to build the reaction sphere (the flask, if you like it) and to place the reagents in the right inlet of your vessel. The rest is "just chemistry" ;) 

I hope i made myself clear enough - at least after the n-th edit ;)

I think the concept is pretty cool.