Do tech companies like Hetzner count as mittelstands?

Mittelstand is singular (like middle class).

I would say decidedly yes for Hetzner. Mittelstand can somewhat be characterized by size, but it's mostly that they are more like a family, have different values and more long term thinking than the larger industry and public companies. It's also kind of a brand that many companies would like to attribute to themselves, even much larger companies that like that seal of quality.

"Mittelstand" in Germany seems to be roughly: below 500 Employees, below 50 Million € turnover. Hetzner makes more like 400 Million € turnover but with 300 Employees. So it's technically above it but they are propably a very tight margin buisness. As such i'd say yes.