The iOS Kindle app has finally made it easy to send an article from your iDevice to your hardware Kindle. Just share it via the share sheet to Kindle and it’ll allow you to either add it to your library or send it to a specific device.
There’s still reasons you might want to use other apps to do this- for example the Matter reader app allows you to consolidate many articles into a nicely arranged Kindle ‘book’. But the basic share feature is great for a one off articles (including webpages, ePubs, PDFs or Word files) or if using apps that don’t provide this facility.