This was the case when PowerApps was first announced; however, this is no longer the case.
While it is technically true that there is no actual code that would be managed and deployed with a PowerApp or Flow but that doesn't mean that you can not use the power of Azure DevOps. Additionally, when creating a PowerApp / flow you would also be creating entities and even Model Driven apps - and these uses solutions - which naturally work well to deplooy within Azure DevOps.
Microsoft is building out this whole construct to enable all these to deploy...
While the whole incorporation of PowerApps and flows into Solutions is not fully baked yet - they are targeting to have this ready around the October time frame this year.
We have been talking to Microsoft about also enabling PowerApps and flows to follow the same expansion that solutions do so that they can take advantage of the full branching strategy.
So even though you would be simply exporting out zip files into your repo - you can still take advantage of the full devops pipeline which is highly recommended.