I've been thinking, so syl mention they both contributed to the bond and got something more from it. With each oath was a different level of contection, Kal (I know it's nothe the full name auto correct keeps changing it) got stormlight, syl got her memories, after the last oath Kal was able to store storm light better and syllabus got a physical for as a shard blade. What if the shard plate was a growth like the parashendi's warform armor. It would be able to store storm light like a radiant and gave added attributes from the spren and it maybe mostly human in origin so it couldn't scream like a shard blade and it would effect lashings because it's a physical part of the person not a worn object.
Another point that shows this is a possibility it the paRashendi comparing them selves tof humans and surgebinders. "We are like broth and the humans are the meat" I believe surgebinders/radiant can bond more completely with the soren. That shard plate is on onlyrics when the spren is inside of the radiant like paras ending in warform and when the radians broke the oath it could have possible separated the mind confining it only to the shard blade. Or the plate is mostly radiant growtha but to a much more complete state than the parashendi warform