Slack Software Engineer, Backend interview questions

I applied for the Toronto opening through someone working at slack. The first round was 30 min HR phone interview comprising of general HR questions. Second round was a 45 min zoom call with the hiring manager. He asked questions about my previous projects and how I collaborated with the other teams. In the third round I had to review a Junior Developer's pull request trying to be as kind and empathetic as possible. The 4th and final round was a 4:30 hour interview with 4 people. The first 45 min session was regarding system design; I was asked to design a chat messenger. The second 45 min session was again with the hiring manager who went a little more in detail into my experience. Next I had a break for 30 min. The third 45 min session was about team work and collaboration. The fourth session was a pair programming session. I was given a link to download code and develop a feature for a private channel. The emphasis here was not on how much I could complete, but on the way I would code in real life. The final 15 mins was a wrap up call with the HR

Applied online through the careers website and was emailed a few weeks later to set up an initial chat with a recruiter. It seemed the the recruiter was fairly new at Slack and couldn't answer questions regarding Slack's engineering teams as well as products/features that I used. It was going well until the recruiter asked about the percentage of time I spent on the backend vs. frontend. Expecting a predominately backend distribution the recruiter quickly told me that he would not be moving forward with me as I told him I was a full stack developer and spent my time equally between both domains. I would say that this is a poor representation of what I thought Slack was, why even setup an initial chat and screen me as a candidate if you intuition is to hire someone based on their answer of how much time they have spent on a technology? Additionally asking a percentage of how much one works on the backend or frontend does not determine their aptitude of that knowledge. I may spend 50% frontend/50% backend but I definitely am adept at both principles.