Why Roman camps do not disprove Grant's mobility argument

Did Roman armies not fortify their camps while remaining highly mobile?

Yes. Roman armies carried some fortification material with them, and their camps differed in size and method.

Roman armies carried part of their fortification material with them. Their camps therefore used partly portable defences alongside timber cut for the purpose. The size of Roman camps also differed from the situation being discussed at Pittsburg Landing. These differences qualify the comparison between Roman fortified camps and Grant's approach to a mobile campaign.