The OP writes that he was threatened to be held in custody (French: "garde à vue"). This is a common trick of the French police: when people are held in custody, they have the right to an attorney and to keep silent, which the police don't like. So the police question "free" people and threaten them to hold them in custody if they don't cooperate obediently. Most people (especially good citizens) are scared of being held in custody, so the trick works quite well: people answer every single question and have no attorney, despite their right (and interest) to do otherwise.
In the present case, the charges (had any been pressed — he was there as a free person) would have been akin to terrorism, which means that the custody would have been under anti-terrorism rules: a very unpleasant experience that can last up to 96 hours. So the pressure was very high, despite his being "free".
Speaking for myself; if MI5 (to pick an example) contacted me as a public and relatively high profile UK editor/sysop and asked me to come in and, say, advise them I probably would.
I keep forgetting we are reading a lot of translated text here; so "summons" might not be the right word in this context. I perhaps jumped to conclusions given the stuff that happened once he'd attended the meeting :)