As mentioned before that announced reason is a bit weird, as there's no staff action going on in the Netherlands. Only bus drivers that are member of the FNV union are on strike, but only in a few regions a day and most drivers are working, so that wouldn't impact the ability of security staff to get to work.
I'd suspect that the huge shortage of security staff at Schiphol Airport is contributing. Possibly security staff working via an employment agency will prefer to work there (as they've negotiated a summer bonus at the airport), resulting in no security staff in the Eurostar lounge. NS, being the formal operator for Eurostar within the Netherlands, has had multiple issues due to not paying enough recently.
For example rail replacement buses often result in chaos at the moment as the coach operators prefer to deploy their scarce staff to more profitable orders.