A bit of context: Apple is being forced by different jurisdictions (EU, UK, Japan) to allow third-party browser engines. In all other regions it's still not allowed to use them.

Apple has been maliciously compliant, putting up roadblocks to testing and distribution. For example, the existing Firefox iOS app cannot be simply updated to use Gecko/Servo; it needs to be a new app.

As far as I know, none of the major browser vendors have released apps with their own engines on iOS. I suppose maintaining two (or more) different codebases for US, EU, etc. is not very attractive.