The Democratic Peace theory has always been bunk: even when it was formulated, for any definition of democracy restrictive enough for the claim "Democracies don't fight wars with each other", the number of available democracy-democracy pairs is so small compared to thr number of pairs of countries that the expected number of wars between them is, rounded to the nearest integer, zero.
Certain, since the wars in the Balkans from the 1990s, the Democratic Peace theory is even less defensible than it was previously.