I think this might clarify the situation:

In the US, unlike other markets I know, a company doesn't go public as such. Rather, a specific block of its shares goes public: a given company may (and typically does) have shares that are registered for sale on exchanges, and shares which are not. Before a given block of shares can be sold, they must be registered. It's not true that one can assume that all shares of a give class are fungible.

If I'm not mistaken, this filing is kicking of the process to get the shares owned by Berkshire so registered. It is a prerequisite to any future sale, and in fact makes the block of shares a bit more valuable, but it isn't any declaration of intent to sell. That being said, it would not be unreasonable to assume that the registration request was at the request of the owner of the shares, and that it does have the effect of clearing an existing hurdle to their sale, be it sooner or later. Maybe Mr Abel wants to sell, maybe he just wants that door unlocked.

Jim