Formal organizing would lend itself to infiltration because these groups tend to be so interested in gaining members that they are unlikely to turn anyone away even if those people show signs of being state agents. Informal groups tend to be much more closed-off.
Authoritarian organizing would lend itself to infiltration because state agents are adept at navigating authoritarian organizations. (It's their job!) Other forms of organizing that do not reflect the structure of the state would probably be much more difficult for state agents to navigate.
Further, authoritarian and formal organizations could lend themselves to be more easy neutralized by infiltration when it happens. In the case of authoritarian structures, if infiltration reaches their higher ranks then these structures are completely compromised. In the case of formal organizations, the effect of infiltration is more widespread, potentially compromising the whole structure, rather than just a circle or a few circles in the case of informal, affinity-based organizing.
I wouldn't say the assumption is inaccurate. I would, however, avoid taking it so far as to have a false sense of security in informal anarchist organizing. Serious damage has been done by state agents operating within such circles.
I was going to say that anarchists have always, by definition, avoided the pitfall of authoritarian organizing, and then I realized this is unfortunately probably not true. In any case, I would say that certainly *some* anarchists have always known to avoid these pitfalls, although their main reasons for avoiding these kinds of organizing may have been other than avoiding infiltration. That said, I think over time and through experience more anarchists have become wary of formal groups, especially large ones, especially ones that put their activist goals ahead of all else and strive for growth in membership and the size of their impact. It will be interesting to see if more anarchists are able to identify and avoid the kinds of infiltrators who have put many of us in prison not through formal groups--infiltrators like Anna, Brandon Darby, and that one in Cleveland.
