Why do we use the doublespeak terms for bitcoin?

There is no "self custody" or "custodial/non custodial" Bitcoin. Either you own bitcoin or you dont't. It is that simple.

If you do not control the keys you have an IOU:

ETFs: you own a promise

Coinbase/Binance: you own a promise

Some other regulated institution/agency: you own a promise

"Custodial wallet/service": you own a promise

There is no such thing as owning bitcoin on an exchange or an ETF.