No, the timing is actually perfect and I don't think that it has anything to do with Bendis. It's more about cross-promotion, which is always huge in media:
1. Sentry and Void got implemented into the rather successful mobile game "Marvel: Contest of Champions" few weeks ago. I think there was quite a bit of traffic surrounding them. They had two events going on for them. I don't know how much money they made. But the developers supposedly messed Sentrys viability in the game up, while Void is a viable character. There was quite a bit of bitching from the player base, that Sentry was a trash tier character in game and that he should get immediate buffs. For a majority of reasons. Even after the developers said that they weren't going to revisit his in game viability anytime soon, people still kept complaining and demanding for buffs. So people care a lot.
2. Sentry was also announced as a card for the Marvel card game "Legendary". I didn't read up all too much on that topic, because there isn't much information, but it seems that there might be more to him than just him being a playable card. I think they might make an event out of it or something like that. No idea how that card game works, but I've read that there might be something special.
3. Sentry was announced for the Doctor Strange comic book and appeared in a couple of issues. Those issues sold surprisingly well. Now the question is if the Sentry played a big part in that, or if he was one of the reasons and the main selling point being Loki and his Sorceror Supreme status. It's also always quite a bit about traffic and what people say on social media. But I do think that there was quite a bit of talk, which then also played a part in the green-lighting of the Sentry solo series.
4. And last but not least... I would assume that Jeff Lemire simply had a good and interesting idea. They wanted to do a book to see how the sales will go (and I personally will be buying 3-4 copies of every single issue) and looked for a creative team. And with Lemires pitch / story idea wanting to take the Sentry and the Void into a new direction and develop their characters and relationship... I wouldn't be surprised if the Marvel editorial immediately agreed.
Oh boy, you missed out on something. "The Age of the Sentry" is amazing.