You don't want concrete to dry out too quickly because that stops the chemical reaction. Concrete will reach about 60% of its designed strength in seven days with full rated strength at 28 days - so you typically want to keep it moist for at least seven days.
As you can see in this chart, the strength gain slows down over time. It will continue to gain strength past the 28 days, but so slowly that it is of less value to work to keep it going.
Clearly, keeping the concrete moist to continuing curing can be an issue in hotter, drier weather. The reaction is exothermic (creates heat), so hot weather plus the generated heat can really dry things out quickly. But you really get no advantage from curing more slowly and you don't want the concrete to freeze - which will also stop the curing. That's why they use thermal blankets to cover concrete placed in winter.
I'd correct the contractor's statement to say "winter placing is actually better because the concrete dries out slower than summer and allows longer curing times."
On compressive strength, anything under 2000psi we consider to just be fill concrete. Structural concrete starts at 2500psi.
Flexural strength is often in the 500psi to 700psi range, though.
I know that people put lifts outside, but I'm not sure I'd do that in an area of regular precipitation or snow. Beyond the motor and electronics, you have the stabilizer and release cables and pulleys. At the very least, I'd recommend regular detailed inspection and lubrication of these components in particular for any lift living outdoors. You mentioned the inspection companies for commercial uses, but few home mechanics are employing such services and it is easy to get complacent about such things - "Well, it worked perfectly the last time I used it, it should be fine."
