As a rule of thumb, web developers do web development and digital marketers spend their time on marketing.
It’s common to see web developers with 20 years of experience who have never had any exposure to marketing (except for working with marketers within an organization). Marketers traditionally don’t write code since it’s usually a different field.
And customers usually pay for digital marketers and web developers separately as the required skill set is different. People profiling in both probably lack skills in each.
Very few people choose to commit themselves to both—usually tech entrepreneurs or freelancers who picked marketing as a necessity to sell their products. It takes a long time to become an expert in each so expect those individuals to have at least a decade of experience (often more).
If you aim for a professional career in either, probably focusing on a specific field would make more sense. It would take you twice the team to become well-versed in each area. Employers want to maximize your efficiency and skills and see your commitment to your job (instead of switching between roles at all times).
Small businesses may be looking for a generalist who can pick up a few bits and pieces from here and there. That said, this isn’t quite professional and you’ll likely learn a little bit of everything with nothing in-depth.
Then again, if your life goal is starting a business on your own (or even running a lifestyle business after hours), this may be a good option. I’d still propose spending several years with web development until you decide whether studying and practicing marketing makes sense.
I have curated a guide on both fields—particularly, a WordPress development guide and a digital marketing guide.