They should be allowed to express their opinion until it becomes tangible and problematic for said outgroup?

Yes. Exactly. That is the fundamental basis of a free society and the legal system. The line is drawn at tangible actions and imminent threats.

You do not get to physically assault people based on hypothetical future crimes you imagine their words might lead to. Using physical violence to punish someone preemptively before anything "tangible" has actually happened isn't justice. Redefining offensive speech as "violence" is just a rhetorical cheat code you are using to grant yourself permission to throw the first punch.

Also that twat called for it... As a crowd gathered behind him, he said: "All of you who throw water are cowards."

"Fight for what you believe in" is arguably the most common idiom in all of political activism. When labor unions say "fight for your rights," or progressive activists say "fight for climate justice," they aren't asking for a literal fistfight. You know exactly what that phrase means.

Deliberately misinterpreting a standard political metaphor as an invitation to a physical brawl is the definition of arguing in bad faith.

Furthermore, throwing liquid on someone is legally physical battery. The mob had already started the physical altercation. He was calling them cowards because they were hiding in a crowd throwing things at a single person instead of actually defending their beliefs with their words.

Your entire argument boils down to intentionally twisting a common figure of speech so you can fall back on the classic abuser's defense: "Look at what he was saying, he was asking for it."