It has, apparently, been retconned, in a way. The source that Kat was giving is more recent and CCP approved and it qualifies the statement in the Mordu's Legion chronicle decisively. Caldari are not xenophobic in every possible regard, as the quotation that Kat gave point out, but "are very protective of their way of life and tolerate only those foreigners that stick to the rules."
Caldari don't have problems to get into contact with foreigners or to have foreigners around, they have a problem with foreigners trying to change their cultural identity (as the Gallente tried to) or to subvert it.
Conversely, Caldari have little scruples to force their way of life onto others: If you're a foreigner who wants to live in Caldari space, you have to do as the Caldari do. That probably counts as well for regions that the Caldari conquer. As I see it, the Caldari aren't subscribing to a doctrine of "Don't meddle in my affairs and I don't meddle in yours." It's a doctrine of: "Don't meddle in my affairs." Full stop. Certainly though they might make it appear a bit softer around the edges, sometimes, but as PF states: "Coupled with the fact that they are more unscrupulous than the Gallente and more combative than the Amarr, this makes them in many ways the most meddlesome of all the empires."
Perhaps surprisingly (or not so surprisingly) the Caldari have very good experiences with the Amarr in that regard. Contrary to what most players intuition is, the Amarr are faithful to the spirit of the
Caldari-Amarr Declaration of Friendship, a pact that agreed to respect the sovereignty,
not interfere in the internal affairs, and share intelligence on the enemies of the other side. At least so far, the Amarr have proven to be valuable allies to the Caldari and vice versa.
For sure, there is the fact that peaceful cooperation isn't the modus operandi that the Empire used to go by in the past. But then the Caldari are probably intelligent enough to realize that in the past, the Empire largely (since the conquest of Athra) dealt with nations that were vastly inferior and that the Amarr realize that the Caldari aren't in that position. (One could even argue that probably the Amarr were only invading the Matari successfully because the Matari caused natural disasters on their home planet by mining away an entire Moon.) The Caldari can be reasonably sure not to get into that position any time soon.
At the same time, the Republic -probably- only exists because of intervention of the Federation. That and the shared history of the Caldari and the Gallente would be enough to cast suspicion at the Republic. But above that the Republic was taking pressure off the Federation and that was reason not to engage in diplomatic relations with the Republic but the Empire, to again increase pressure on the Federation.
So, 'historically' speaking, I see little reason for the Caldari to rethink their alliance with the Amarr or try to approach the Matari. Also, the Matari wouldn't be assured to live with any less outside influence if they switch from the alliance they share with the Gallente to an alliance with the Caldari.
To me the conclusion is that culturally, politically as well as historically, there's a gulf between that Matari and the Caldari that seems to be insurmountable. Exceptions to the rule are just that: exceptions. I as well think that the Caldari capsuleers aren't in general as friendly to the Matari as it seems. Those friendly to the Matari only seem to voice their opinions more loudly.
So, all is well, imho.