When companies give the reason for turning you down for a job, there's a lot of disincentive for them to give the honest precise real reason (if there is one). There are many ways to be accused of hiring discrimination, whether ageism, sexism, ableism, etc. Basically it's unsafe / unwise for an employer to tell the truth here because it can lead to negative consequences for them, even if their hiring processes are on the up and up.
Similar to when a VC turns down a startup for some reason like fit, I definitely would not take this feedback at face value. Also, culture fit is a generic catch-all bucket for basically anything the company wants besides technical skills, and it may not be well defined.
As you have mentioned, I've also seen less good developers take the roles that better devs should have because they fit into a company's culture more, however that was defined. You probably don't want to work for those kinds of companies. It seems like you're more interested in a culture that values focusing on hard technical work and I think you should seek that out more explicitly.
Keep some good side projects going, have a good reason for your gap time between positions ready for interviewers, and try some mock interviews to get authentic feedback. You can do a free practice interview on Interviewing.io, and I think Pramp is another similar service.
P.S. Don't forget that hiring in our industry is broken. Interviewing for a technical position is a separate skill from developing software. Interviews should focus on making sure a dev would be good at the tasks they'd actually do day-to-day, but often they don't. It's not ideal, but it's just something to keep in mind. Sometimes at smaller cos (startups) it's possible to get hired in other ways and circumvent the traditional interview process.