I only use the marks on my mats for rough measurements. Like if I need to cut a half yard off of a large piece of yardage to use for a certain project. I don't feel like it's as accurate to use them since there isn't a ruler pressing on them to keep them still, if that makes sense.
On the same note, why does she say to press and not iron back and forth while she is actually ironing back and forth?
eta: I have not seen the videos the OP is talking about, but if she isn't adding a disclaimer that mats may not match rulers or that all mats may not be accurate, it could be a frustrating lesson for some people that don't know. Like beginners.