A professor invited me to their seminar when I enquired about working on a master thesis with them: did they reject me?
The first one has clearly rejected you for the reasons he gave.
Their seminar group is a place where their existing students might present and discuss their work, including PhDs and post-docs. This invitation essentially is for an initiation ceremony. If you like their work and are interested in making a project out of their research directions and they think you have the requisite background they might accept you in their group and the professor will work with you for your thesis. That is what I make out of this.
Be positive and enjoy a nice seminar. One should worry about things in their control. Best of luck!