I think a big reason as to why it gives off Portal 2 vibes more than Half-Life is due to the constant dialogue between Alyx and Russel which is similar to Wheatley/GLaDOS in the sense that there's always someone talking to you throughout the entire journey. For me, this duo dynamic felt more similar to Firewatch's Henry and Delilah.
Another thing that plays into that Portal 2 feeling is the use of music, especially the dynamic aspect of it. Half-Life has always been similar to something like Samurai Jack, where you have long moments of background noise being punctuated by music at key intervals. Of course, the fact that Kelly Bailey hasn't worked on this was heard throughout. Music definitely feels more Portal 2-like, with a clear contrast when they use the old hl tracks now and then.
However, I would say it made up for those aspects with the more horror-like experience and macabre depictions of Xen and combine. That was very much in line to what we've seen in previous Half-Life games, maybe even more than in HL2.
But deep down we all know what it was truly missing...