No.
Undermodified malt requires some special techniques to get good extraction and efficiency. Undermodified malt is actually pretty rare these days; you have to special order it, generally, unless you order from one or two big online houses. The overwhelming majority of malts are very well-modified, suitable for single-infusion mashing.
Step mashing is just another tool. A single tool can be used in a variety of ways. For example, a claw hammer can both pound nails in and pull nails out. A step mash is a method by which certain functions are performed in the mash. The functions may be required/desired for a variety of reasons.
I should have written, "It's not undermodification that makes a low-temperature rest a good idea in this specific example." I did not intend to implicitly or explicitly state a generalized rule.
Does that make it more clear?
Bob