I'm not familiar with that particular trade itself, and this is just based on general knowledge of baseball, albeit many years of being a baseball fan and patron of The Show, so take it with a grain of salt:
Players who are on the 40 man roster after September 1st are considered on the major league roster and cannot be traded (traditional trade deadline has passed). Minor leaguers who were not on the 40 man roster on Sept 1st can, however.
Teams are therefore able to trade minor leaguers all they want.
Trade waivers for MLB players is something that I never fully understood, but it at least helps address your concern. It seems as though the team placed the major leaguer on trade waivers which allows other teams an opportunity to claim him, which almost never happens; or at least a claim is almost never completed because the original team can take it back (like I said, I don't really understand that aspect of it). If he goes unclaimed, he is then removed from the 40 man roster and is available for trade.
At least that's my general understanding of it.
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