Red Nova Labs Full Stack Developer interview questions

The process was generally professional and fast. There were two rounds of interviews. The first round was with a Senior Software Developer and their CEO, and it was a mix between behavioral and some knowledge-based technical questions, touching the surface of projects, web technologies, data structures, and algorithms. The second round was their CTO and it was a little more in-depth than first round but wasn't difficult. Just know you stuff and you'll be good. We moved forward quickly and they extended an offer a day after the second round of interview. We agreed that I will have two weeks to accept the offer, but the next week they sent an email saying they found another candidate, and thus my offer letter was invalidated - even though the offer letter's deadline explicitly said it was due in two weeks. I personally have never experienced things like this. While it is perfectly legal to do, I doubt it is really ethical/professional to rescind offers like this.