Sounds like it is one of 2 things, either a sales speil from a salesperson so they can sell highchairs or their particular highchair. The other, if there is a specific reason for a certain child to ensure that their feet are flat and not dangling. Often people will take the advice given for one child with special needs and apply it to all children.

I think you would find it hard to find a highchair that allowed the child to sit with flat feet for the whole time they are in the chair. They grow and unless the foot rest was adjustable, their legs are going to be too short or too long most of the time.