"Equipped" for IFR and "suitable" for IFR may be 2 different animals...
While a set of certified "standby" instruments may be all that is required by the FAA, they may not be suitable for extended IFR flight as a powered airplane. Smaller instruments may induce eye strain and may not have the fidelity and quick-look capability that standard instruments have. OTOH, they may be fine for inadvertent IMC or emergency use (their intended purpose).