What WHBM was referring to was devices whose physical speed is governed by synchronous AC motors. Their speed depends on line frequency. If you have an old non-quartz clock, a phonograph turntable or an 8-track tape player that works this way and was designed for 60Hz, and you run it on 50Hz (assuming you deal with any voltage issue), it will run at 5/6 of its design speed. That will make its music/timekeeping/etc. fidelity less than you might wish for.

Since modems don't use synchronous AC motors, they don't have this problem. Their "speed" isn't the physical speed of anything. It's an electronic frequency. Totally different technology. Has its own issues, but this isn't one of them.