I find this discussion fascinating. When I hear people advocate for SLOs (and SLIs) they are often quite rigorous in how they approach it... that is until the very last step where they hand-wave the math and produce numbers that don't mean anything (like averages of percentiles and such). Often times (specifically for sites/services that have undulating traffic volume like more users in the day than at night), the incorrect mathematics can produce wildly inaccurate outputs... so all that rigor and you end up determining that you've failed or succeeded when, in fact, the opposite is true.
I appreciate the rigor in this post because it provides clear and simple instructions on doing the last step (your calculations) correctly so that all those fancy SLOs are actually honest.