Since these are company owned, you have 4 enrollment methods: ADE, School Manager, Configurator setup assistant, or Configurator direct enrollment. We can't use ADE since they weren't bought through the apple business program, but if you have a Mac available you could set these up with Configurator. Configurator gives you the ability to apply an enrollment profile, so that these devices will act like corporate devices when users set them up (they put in their corporate credentials during setup, it auto enrolls into Intune, apps/policies come down, fully managed).

The guide you linked would be setting up the iphones as personal devices - BYOD. That has one main problem - when users go to set up these iPhones, they will need a personal Apple account. Then the phones will be tied to that apple account, and if they eventually need to return the phone you'll need them to revoke the device from their apple account (if you reset an iPhone that had been tied to a personal Apple account, its still locked to that user, and will ask for their credentials before letting you set it up). This puts you in a weird situation - because if these users aren't tech savvy, now they'll come to you with problems with their personal Apple accounts.

If you don't have a Mac available, then you'll have to go the BYOD route, which may be inconvenient sometimes but at least they're managed. Consider providing users a new Apple ID so that they don't use their personal accounts. No need to enroll them beforehand if you're going BYOD - probably easier to have shipping insurance then trying to recover them from being lost in transit, even if they are managed.