Must the authors of Webix publish the non-concatenated source code as well?
It's kind of a grey area.
The GPL mandates that the work must be conveyed in sourcecode form. Obfuscated sourcecode is still sourcecode which could be edited when one is masochistic enough to do so.
But on the other hand, the GPLv3 defines "sourcecode" as follows:
The “source code” for a work means the preferred form of the work for making modifications to it.
Artificially obfuscated sourcecode is not the preferred form of making modifications when the licensor also has the unobfuscated version available.
Regarding your questions "Must the authors of Webix publish the non-concatenated source code as well?": Actually, they do offer the unobfuscated sourcecode. But for the sake of argument, let's pretend they would not.
When they are the sole authors, then they would not break any license. They can publish their work however they want. When the GPL is violated, the only one whose rights are violated in a legally relevant way is the copyright owner. As a downstream consumer, your only right is to decide to not use their software when it does not fulfill your quality standards. The only people who might be able to claim a violation of their rights are any upstream contributors whose work got included in the software and then got obfuscated to an unreadable form. Whether the obfuscation is a violation of their license terms or a permitted form of modification is for a court to decide.
However, it is questionable if it makes sense to publish something under GPL and then discourage people from creating derivates or contribute patches by obfuscating your sourcecode.