That depends on your requirements and constraints.

"Designed with web technology" doesn't necessarily mean "designed for the public web".

In many enterprise settings desktop-only is perfectly fine because applications developed for these environments often won't ever be accessed from a mobile device anyway. Even if they are it's usually only a small part of the application that needs to be available for mobile devices.

In these cases, devoting an inordinate amount of time to making all of your design work flawlessly on mobile devices can mean a huge waste of resources.

By the way, from a cursory check - while not particularly mobile-friendly - this UI toolkit is responsive (probably also because that makes perfect sense on the desktop as well) and seems to work alright on mobile devices, too.

The web is a deployment technology as much as it is a display technology. Even when you design something that's only supposed to be displayed on desktop screens, it may still make sense to use web technologies if it makes your deployment easier (e.g. because the target environment is heterogenous, with lots of different OSes).

There are lots of enterprise and other complex webapps that will never be used on a mobile browser, or have accompanying mobile apps for that scenario.

There's no need to be good at something that doesn't apply to your situation, especially considering how much more work mobile UI takes.

E.g. we have an in-house web based ERP/CRM. Nobody is allowed to use phones at work and no one works from home. Supporting mobile is useless for our project.