It absolutely is. If security is standing in front of the door to the stage are they going to push past? How is security supposed to remove them from the stage without using force? It's force that's designed to make the protesters look like they're the victim if anyone tries to get in their way.

A peaceful protest would be standing outside on the sidewalk with signs.

You're importing protest tactics developed for use against governments and large corporations into a context where they don't make any sense. There, this legalistic distinction between "force" and "disruption" is quite important, because ensuring you don't cross the line to "force" greatly decreases the legitimacy of any violent response. It doesn't matter at FOSDEM, because FOSDEM is not going to hire goons to beat you up regardless of what you do.

>> Disrupting access with your physical presence is not a use of force.

When they try to move him, will that be use of force? When he tries to resist being moved will that not be forcibly staying?

Yes it is. You basically being a fascist by turning up using physical force to block free speech

You invoke violence by disrupting somebody elses rights. You force people to use violence/force to remove you in order to maintain the right to free speech.