I'd like to add on to @Anatomy5803 above with some more notes
Open sourcing Orion, from a management point of view, would move the existing teams manhours from active coding to vetting code submissions.
In the time it would take one coder to write one feature, they can review the work of multiples more coders. Even if you assume that 80% of pull requests do not meet the standard of the project, it would take the same amount of time to get the work done with significantly less cognitive load from your team.
Through a process of interacting with open source contributors, you can utilize your trusted team to create more trusted contributors. It is true that there might be a dearth of talent initially, but as this grows - you will effectively be training contributors in WebKit.
Secondly, the increased visibility that comes with open-sourcing the project will open new avenues towards donations/funding. Contributors to open source projects tend to be zealous advocates once they get going.
Further, IP can be protected through several means. I'm sure Kagi inc has already trademarked the Orion branding - but one can go further and patent the implementation of the browser itself. If the project becomes open source & key aspects of the browser are patented there is little risk of IP theft.
To add onto this, it is very important to license the project under something like the FUTO license that aims to be copy left for the source code, but restrictive on the monetization (which is the ultimate goal anyway). Perhaps a custom license modelled on an existing license like the AGPL could be useful.
Lastly, security vulnerabilities can occur in systems that are extremely well designed even. A browser being the gateway to the modern world should require security at the same level as an operating system - especially given how complex it is. More eyes here the better.
Some of the largest open source code bases that exist today started out as projects by a single person in a basement. The biggest failures have been because of failures in resource management and funding to keep the project going.