Can anything besides Bitcoin be considered DeFi?
Obviously yes and Ethereum is a perfect example. The bitcoin maximalist don't give Ethereum enough credit for its contribution to the ecosystem. I strongly believe in bitcoin and believe it should be the first and go to coin when cryptocurrency or decentralized finance is concerned but that doesn't mean I should hates on other platform especially those bringing utility into the Industry.
If I may ask what does make Ethereum centralized, I know you'll immediately reference the influence the founder has on the blockchain and the number of nodes plus who controls it. I won't disagree with you that the nodes currently in existence aren't that ok for a decentralized project but still that doesn't mean you should discredit the effort Ethereum in putting into giving us rhe best decentralized experience so far in the industry.
All that happens on the bitcoin network is just making the blockchain boring and been overshadowed by whales making a living off noobs. For now bitcoin is a currency while ethereum is a platform. The sooner the platform developers starts working more on its platform features the best it'll be for the blockchain.
You certainly have a point here, Ethereum does bring utility into the cryptocurrency ecosystem. Being centralized (or at least, less decentralized than Bitcoin), innovations in Ethereum are happening more frequently, and different features are being embedded faster than it is in bitcoin. Ethereum platform allows developers to test their ideas and build apps and financial products. In a nutshell, due to Etereum's centralized nature, changes in the platform don't require overwhelming consensus to be added, and consequently, it became the perfect solution to the DeFi problem. However, there is a small issue, and that issue will likely prevent Ethereum from being the leading platform for DeFis.
DeFi products are meant to replace (or to become an alternative to) the current centralized banking products, traditional services that are based on fiat system with many drawbacks. To build truly decentralized financial products and get rid of the disadvantages of government-issued money, I believe that we must use time-honored decentralized blockchain and bitcoin, which has proved it has credible monetary properties.
Ethereum, on the other hand, has neither of these important properties. It is so centralized, it has so few full nodes, that governments can easily shut it down if they suddenly decide they don't like alternative financial products. Ethereum itself, as you said, is not a currency but a platform. Its monetary properties are not credible, the total supply is unknown and hardly verifiable. Neither can be credible the tokens built on this platform.
We can test features and try to innovate on centralized blockchains, but we can't rely on them when it comes to our money.