The "problem" is the printer doesn't output either sRGB or Adobe RGB (1998) but rather the color space of the profile they presumably sent you. I say presumably because unless they use that actual profile to convert the data (and we don't know exactly how), assuming using it for soft proofing only is a stretch. The ideal CMS workflow would for them to supply you the actual output profile for the device, you soft proof and select a rendering intent you visually prefer, then you convert that data to the output color space. Maybe with additional output specific edits based on the soft proof/conversion. If they don't allow you to convert the data, you're not using the system as it was designed. Nor do you know if the profile supplied even reflects the output conditions at the time the data is sent to the device. Might be close, might not be. But not ideal.