astromodified camera = camera in which the infrared cut filter has been removed so that an extended spectrum in the infrared range can be recorded. The Hoya IR72 filter lets IR of 720nm through and filters out the visible light. This IR range is more for effect photography in daylight. The aforementioned IR cut filter lets IR of 720nm through, so that this range can still be captured at all with an unmodified camera. The human eye can also just about perceive this wavelength: Just hold the IR72 against a light source (lamp - NOT against the sun!) and you can just about make it out.
In short: Photographing with an astromodified camera and IR photography with the Hoya IR72 are two different things, i.e. astrophotography at night versus IR effect photography during the day ...
Certainly, you can also take pictures of the Orion Nebula with the Hoya IR72 filter, you just have to expose for a correspondingly long time (only possible with tracking), so you get a picture of the Orion Nebula in the range of IR 720nm. An astromodified camera would provide an image of the Orion Nebula in the visible range including IR - beyond 720nm.

This filter is not a substitute for an astro-modified camera. I also don't know how effective the filter is when you put it on a normal camera, as these have an infrared filter.