Well, business is different; you don't have to use your phone for that, you can use a separate smart device (which may be a non-phone like tablets or perhaps iPod might work) just for this purpose; and, unlike consumers, there's generally no taboo in requiring businesses to fulfill requirements which may need extra hardware that they don't yet have and need to purchase, or require them to change their buildings, etc, so saying "your business needs to run this app" does not imply "my phone needs to be iOS or Android" as there's zero expectation that your personal phone should be sufficient for that requirement imposed on the company.

Also, they're not saying that your business needs to run this app, they are saying that your business needs to verify Covid certificates and offering these apps as one way to do it. But it's not the only way - for example, it may be less convenient, but you can do that QR code scanning + Covid certificate validation on any computer with a connected camera using a web service (https://app.digitalcovidcertchecker.gov.ie/ is one random example, there is also open source code to roll your own checks for EU Covid certificates in some custom system if you need to), and there you can use Windows or Linux or whatever, there's no need for a smartphone as such. In fact, if the parent poster's Pinephone can run a browser and expose a camera to it, then perhaps it might work out of the box on the Pinephone without needing a special app.