Does Teemo Strategist work on all attacking units while he is defending?

No, Teemo, Strategist does not work on all attacking units.

According to his card text and the RiftJudge FAQ, when Teemo defends, you choose only one enemy unit at his battlefield to target with his ability. The damage is then dealt specifically to that single chosen unit based on the number of cards with [Hidden] you reveal.

As per RiftJudge FAQ #8586: "No, Teemo, Strategist cannot target multiple units. You choose only one enemy unit with his ability, regardless of how many cards with [Hidden] are revealed."