I can't say for Google, but at work it's more or less how it works at the office (mostly software dev, half a team does some firmware/hardware), but it's more ad-hoc than as a rule. Like all the teams are small, all the TLM equivalents started as devs before being promoted to their management position, so they have time to do some technical work; how much and what technical work depends on the team, some are still directly contributing to the team's products, others are more on (technical) ancillary tasks, which can be interrupted by management questions with less impact on the development.
I find that it works well, the TLM keep a foot in the action, so to speak and has a better idea of what's happening with the product being developed, what issues we're facing (also in terms of tools, environments...) and it keeps their knowledge of the product more up to date. Of course with their background, I wouldn't say they are all the greatest at managing, but I don't think they've ever done big mistake on that side of their role. So in short in our case it works, but it could just be a consequence of the local organisation and people working there.