I am fairly certain you have misinterpreted those sentences.
The mention of updates and upgrades is because the 50% off sale was initially just for new Cubase licenses. After feedback, Steinberg decided that updates and upgrades would also be included in the 50% off sale, though this took a little time to arrange.
The mention of those who have already updated to Cubase 10 is that the newsletter included a 30% off sale on Halion 6. There are other offers coming over the next few weeks that are intended to offer something to those who already have all the Cubase 10 licences they need - I believe it is WaveLab, Dorico and Iconica that have been mentioned.
If the OP has not yet activated their Cubase licence, they might be able to back out of the sale within 14 days, then repurchase at the sale price. If the OP has activated their licence, it is almost certainly just bad luck. There is always the possibility of a discount coming along in the future. You pay the asking price for the product at the time. There is no guarantee what the price will be tomorrow. It is well known that Steinberg have sales from time to time, often in the late spring / early summer. You can buy a Steinberg licence and wait to activate it until the grace period for the next version has started, which gives a free update to that next version on release.