These are not normal account signups, they are signups to become an associate member (https://join.osmfoundation.org/) which can vote in foundation elections.
Normal members can participate to the election too, according the website. The differences between regular and associate member are 1) the associate’s name and address is not publicly inspecatble, while the regular member’s is; 2) Associate members cannot vote on resolutions.
Sure. I think the point is that these aren’t OSM contributor “signups”, they’re Foundation memberships. The Foundation is not as big a deal in OSM as it is in Wiki(m|p)edia because the project is intentionally low-budget and hands-off.
Even that is not really comparable. The WMF does not actually have any members, and only has a superficial pseudo democratic shim (if that is good or bad is a different discussion though).
The "alarm" is caused because concerted action of roughly 10% of the OSMF membership could easily tip the current wobbly balance one way or the other, and there is no guarantee that the current "hands-off, low budget approach of the OSMF would survive such an upheaval.