Not true for iPhones - they have all that’s necessary to create a full strapdown INS. I’ve had iBFD running for a whole flight including aeros and IMC and it was exactly in sync with my EFIS. That particular app I think was written by someone who writes software for instruments for a living and uses the gyros, accelerometers and GPS. If you don’t believe me, download it and try it - not expensive at £12. It’s not certified in any way but it’s better than a blank screen inside to go with the one outside!
I’m not suggesting for a minute that you should strap it to your yoke and take off in OVC000 and 75m but as something that is there for use in an emergency it could save your life...