Well, the expert to whom I believe you are referring was correct in the main, but remember the context.
He was testifying only about specific pictures recovered from a Helio phone.
EXIF data apply to digital images, and within his testimony the discussion was bounded by the JPEG compression image type.
EXIF data are embedded into captured images whose digital data are then written to files by devices such as scanners and digital cameras, including those contained within other devices; e.g., phones, laptops, tablet devices, etc.
See if you recall his testimony sounding a little like this:
"EXIF is short for Exchangeable Image File, a format that is a standard for storing interchange information in digital photography image files using JPEG compression. Almost all new digital cameras use the EXIF annotation, storing information on the image such as shutter speed, exposure compensation, F number, what metering system was used, if a flash was used, ISO number, date and time the image was taken, whitebalance, auxiliary lenses that were used and resolution. Some images may even store GPS information so you can easily see where the images were taken!"
Source:
http://exifdata.com/
What he stated about digital data being unalterable is, of course, not true.
Digital data may be altered by anyone who has the wherewithal.