Method, and what it does not show
Klaviyo assigns a different address on every send, so a sender's pool is a set rather than an address. Brands whose sets overlap by 60% or more are grouped. The percentage on each pool is the mean overlap between its members, so a loose grouping reads as loose.
99 brands had enough observed sends to test and matched no pool, and 20 more have been seen too few times to judge at all. That is a statement about how much of their mail we have received, not evidence that they send alone. Most senders are missing from this page, and the honest reading of a pool is “at least these brands”, never “only these”.
Single-link grouping chains: below a 0.4 threshold every brand here collapses into one block. It is stable between 0.6 and 0.8, and 0.6 is what this page uses.
None of this is in DNS. Every Klaviyo customer on the SendGrid network shares one DKIM CNAME target, and the per-customer zone carries only a key and a generic SPF include. Pool membership appears in received mail or nowhere.