Looking at things from Johnson's perspective, there are three different ways this can play out:
1) he signs an ELC with Buffalo that starts this season, with him in the NHL getting into the last game.
2) he signs an ELC with Buffalo that starts next season, and finished the year in Rochester on an ATO.
3) he doesn't sign an ELC with Buffalo and becomes a free agent in August.
Since he has been in negotiations with Buffalo, we can probably rule out his having decided already on option 3. Also, from what we know, he wants option 1, whereas the Sabres want option 2. Let us assume for a moment that he had already decided that he is going to sign with the Sabres, regardless of whether it's an ELC that starts this year or one that starts next year. Even under such a scenario, it makes sense for him to not sign yet, provided that the Sabres are still not offering him the chance to finish out the season in the NHL. Since option 1 is the one that expires first, and it is also the one that Johnson seems to prefer, it makes little sense for him to cave in while it's still on the table. Not only does agreeing to option 2 preclude option 1, the threat of free agency is the only real leverage he has to try to burn that ELC year.
So even if he is 100% committed to becoming a Sabre, it doesn't really make sense for him to sign yet, if the team does not give him what he wants, especially as option 2 will still be available after option 1 is no longer possible. After all, there's no particularly good reason for the Sabres to refuse to sign him if he holds out as long as possible for option 1.
That's not to say that things couldn't backfire on the Sabres. It's possible that negotiations going south could make Johnson decide that he's rather take the free agent route. It's also possible that he has already decided that unless he gets that ELC starting this year that he'll test free agency.
Regardless of how things end up playing out, though, I would be very surprised to see him sign his ELC before the end of the season, unless it's an ELC that starts this season. That said, if he has not signed his ELC by the time the Amerks start their first playoff series, then I would expect him to test free agency.