Soon, your statement will be false due to Metamask Snaps feature. Basically it works like browser add-ons/extension in order to support blockchain not supported by MetaMask itself. I say soon since this feature is still on open beta.
Looks like anyone can develop and publish add-ons. The Solana wallet feature is developed by Solflare itself[1]. I wonder if this means they're confident that their native wallet won't lose customers, or is this a tactic to expand their reach? Looks like they developed this in 2022. Hopefully, it won't be plagued with fake plugins when it's officially released. CMIIW.
https://snaps.metamask.io/snap/npm/solflare-wallet/solana-snap/