Normally I'd agree, but the timing just seems pretty gross.
So they shouldn't have provided a discount and should have just kept selling it at full price and that would have been
better somehow?
App developers on the App Store are stuck between a rock and a hard place. Apple provides no ability to offer upgrading pricing, so developers will either have some people disappointed by having purchased an existing version only to find a new version released a few weeks later, or they have to pull it from sale during that period (and then enable pre-ordering for the new version, if that's a thing that's available on the Mac App Store like it is on iOS, and have people complain that "pre-orders for apps are stupid", while simultaneously making it impossible for anyone to buy and start using even the existing app right away if they wanted to).
If they were to pull it from sale, how long do you feel would be a reasonable time frame for them to have it unavailable for? Obviously longer than a few weeks, since you're suggesting that their providing of a discount and then 7 weeks later releasing a new version was "pretty gross". Should they have it unavailable for months prior to releasing a new version?
The reality is that there's no good solution unless Apple provides upgrade pricing. Even then, someone would complain that they just bought the existing version and now have to pay more money for an upgrade.
And blimey, we're talking about
$10. How much do people spend on their phone bills alone, every single month? If people are in a position in life that they are so cost-sensitive that spending $10 for a new version of one of their most used apps is an issue for them, they probably shouldn't be buying any apps at all, they should be concentrating on how they're going to buy their next meal.
If a third party Twitter client is important to you (it is to me), be thankful that anyone is even developing one given the tenuous economics of being at Twitter's mercy. They certainly can't be expected to keep doing it if they can't charge for new versions.