Inside information doesn't have anything to do with market manipulation. The trader who caused the Flash Crash in 2010 was similarly prosecuted for artificially manipulating the markets. If Softbank was buying options to somehow manipulate the rest of their trades, I can't see how this isn't illegal.
2010 flash crash was due to spoofing. Here SB bought call options outright. Nothing illegal about that.
Market manipulation is also separately illegal. Though classic pump and dump doesn’t seem possible with amzn.