Ok, so I've found that CAD Exchanger is able to open it as non-mesh (B-Rep) solid rather than the mesh objects I was getting with the AD online viewer and the Fusion hub viewer. There seem to be various potential online CAD conversion services that might work but I wasn't willing to sign up or pay to see whether they worked on your file. CAD Exchanger is able to export it as STEP and various other formats. Unfortunately CAD Exchanger isn't free but it might be worth it to you.
Here's your model exported as STEP then imported into Fusion.

As @TheCADWhisperer mentioned: there are a couple of slivers in addition to the main body (highlighted below in blue).

These bodies seem to be duplicates of two of the fillet faces on the main body and deleting them doesn't appear to be detrimental to the model.

I don't know whether any of the above is at all useful but hopefully it will be of some help. I've attached a STEP file in case it is of any use.
If this answers your question please mark the thread as solved as it can help others find solutions in the future.
Marcus Wakefield
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I've created a Windows application (and now Mac as well) for creating custom thread files for Fusion. You can find out about it here. Hope you find it useful.
If you need to know how to offset threads for 3D printing then I've created a guide here which you might find useful.
If you would like to send me a tip for any help I've provided or for any of my software applications you've found useful, you can do this via my Ko-Fi page here.
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