It's definitions and columns that folks are grouped in. When asked which party to you associate, identify or affiliated yourself with, Democrat, Republican, independent, other. that determines which column one falls into. Then the second question, who did you vote for. So it comes out on average 92% of those who say they're Republicans and Democrats vote for their candidate. Sometimes more and sometimes less. Only 89% of self identified voted for Hillary while 95% voted for Biden. Swing voters have their own column. By contrast 88% of republicans voted for Trump in 2016, 94% for Trump in 2020.
You're absolutely correct, but that 8% isn't identified as swing voters, just party faithful who didn't vote for their party's candidates. Semantics, possible and probably so. Now these are presidential election figures. There's also nothing saying that the 5% of Democrats who didn't vote for Biden, didn't vote Democratic down ballot. We don't know as the exit polls, didn't break it down that far.
Numbers are interesting, they don't provide all the answers, but probably do for most or a good majority of the answers.