Hi sipa, segdata differs from segwit in that segdata is unspendable, not required to be download at all during sync or to be retained if opted out of, without affecting block validity. Witness still imposes the IBD and unpruned storage burden. To quote the BIP:
Where SegWit separated signatures (validation data not needed once the containing block is buried) from transaction identity, SegData separates application payloads (arbitrary data not needed once the containing block is buried) from both transaction and witness regions.