Conceptually, if we start from the notion that authoritarians value authority, order, and/or rule over freedom, that authoritarians value obedience over autonomy, then anarchists are anti-authoritarian by definition - no matter if the authoritarian manifested is a person, policy, or practice.
However, in the messy reality of anarchist practices you still see a lot of confusion and good intentions gone awry. In the realm of practices there is no group that can universally be said to anti-authoritarian. Real people are not saints. Real people are fallible. It is easy to point to something that tramples the wills of people and oppose it. But opposing the bad guy, the boss, the dictator is easy and not very deep. If this is the extent of the analysis of authoritarianism, that it picks off particular people or programs, but leaves intact the structures that they plug into then this easy type of anti-authoritarian stance is below the anarchist bar.
If we start from the practices of people or groups that call themselves anti-authoritarian we get a much different picture. In the Greek uprisings there are groups that identify as anti-authoritarian but they are not anarchist. In the US, Maoist and Marxist groups claimed the anti-authoritarian moniker as their own. On the left the anti-authoritarian sloganeering pedigree gets good mileage on all sorts of causes, including presidential elections (meaning non-anarchist causes). So, like in the example "all squares are rectangles, but not all rectangles are squares" we can see that all anarchists are anti-authoritarian, but not all anti-authoritarians are anarchist.