I have a related degree. Sorry, but no. There's a high probability it's within a few percentage points of that number.

This isn't an ABC poll. It's a page on ABC's website which tracks polls published by polling companies.

The 45% is from a polling company with a sufficiently large sample size. This isn't just one isolated poll. You can keep scrolling, and you'll see similar figures from multiple polling companies. The chance that a hundred polls are all wrong, is very low.

You may have heard of YouGov, a UK based polling company with a good reputation. As you can see they have a poll they published last week which has Trump at 46% favourable. If you click on yougov it'll link you to the full poll data, so you can read about their methodology, and how they weighted things so that the sample is as representative as possible.

And yes, the good pollsters weigh the data so it better reflects the population. No, they don't only phone old people anymore. Those days are largely gone, outside the occasional partisan poll.