Nonsense. Does he have to manufacture his own hardware and make the dyes for his paint to for it to count?
My definition of a custom drum builder is anyone who offers a completely custom building service. That's a company/individual who offers to provide any size, finish, construction you ask for, c/w a choice of shell hardware etc. Within that definition, whether the builder makes their own shells, shell hardware, etc is irrelevant. This is not a quality statement or bar to reach, it's just a classification. For the record, I do not regard my company as a "custom builder" for the reasons stated above.
All that said, there's different levels of builder/manufacturer, some offering custom builds, some offering defined series of drums, & some offering a level of customisation of defined series drums.
Making your own shells is often used as a differentiator between those who buy parts in from others & assemble them, compared to those who craft their own shells. Even amongst those who make their own shells, there's very few who make their own shell hardware too. If you use outsourcing as a differentiator, that encompasses 99% of the business at one level or another.
Of course, ultimately, it's the finished result that matters. There's a ton of builders & large companies alike who give the impression they make everything themselves, or at least the key components, but the truth is very different. Does that automatically mean they produce a lesser product as a result? Usually, the answer is no. Outsourcing is a decision taken on a financial basis. The constant drive towards the lowest possible price. Ok, maybe that in itself tells you something about the companies concerned, but the ethos varies massively from company to company.
There is a school of thought that companies who make their own shells somehow have a better understanding of the instrument's design. Although not always directly the case IMO, there is some "hands on" truth to that. Only by working closely with design & production of the various elements, including shell hardware, do you get a full & close appreciation of what works & what doesn't. Again, not a quality observation, more of a human nature observation. Sometimes that very much feeds into the finished instrument, sometimes it doesn't.
Finally, the most important aspect of all - knowledge, & the design ethos of the company. A holistic approach to cohesive instrument design, backed up with an intimate knowledge of how design elements/choices interact to produce a focussed result is going to deliver a well performing instrument. Many drums are not the product of such a process, often, with other considerations entering the mix, but little of that has anything to do with levels of outsourcing.
The worst kind of "custom builder" is the individual or group of people who offer an assembly service with little - no real knowledge of how the instruments work. Essentially, "we'll build anything you want, no matter how crap the outcome may be, & irrespective of how it will sound/perform". Access to that market is low investment & immediate, & that's why many builders who really know their craft use making their own shells as a differentiator. Shell making can be easy in the case of ply shells (but requires investment), or difficult in the case of other forms (but requires skills to pull off convincingly). Either way, it's a barrier to entry. So, although making your own shells isn't necessarily an indication of instrument quality, it can be an indication of how seriously the business is taken, & that's likely to equate to a more focussed product range, & greater knowledge than someone armed with a work bench, spray gun, router, & a battery drill. Of course, there are notable exceptions too.