Non-captive, threaded and interference are the only options to for those bolt. To me interference is the best option given the dissimilar metals (rules out welding like the OEMs would do)
If a threaded fit was used I would expect to see screw in studs not bolts.
If it was an interference fit I wouldn't expect to see off the shelf bolts with a washer used. I would expect to see bolts with a round flat head and an over sized shank, interference also wouldn't require the nuts to be installed for photo's.
You can also see in the photo's that it appears as though the holes for those bolts are elongated. which rules out both of the captive options.
It's really only an installation issue, once installed it doesn't mater anymore.
Just flick them an email asking the question