But here we're talking not about self-aggrandizing nonsense like that, but about his lies that China is paying for the tariffs he imposed on Chinese goods. While we all know that in reality, the precise breakdown will be different in every instance, but in any case it will be the American importer who pays the tariffs directly, and also indirectly, in most of the cases, the majority of the costs are likely to be for American companies and consumers. Sure, there will be cases where the Chinese exporters reduce their price in response and so compensate a certain part of the tariffs, but it's only if they're really very reliant on the American market and can't easily find buyers elsewhere, that this would amount to very much.
Looking into it further, the claim appears to be based on a settlement reached a few weeks ago between California and an organization called Judicial Watch, in which California agreed to update its lists of voters by removing over a million inactive voters (apparently it had been decades since they'd made such an effort to remove inactive voters, hence the massive number). See link. So that would explain where the massive number comes from, but of course, the whole thing about them being inactive voters is that, you know, they didn't actually vote in any recent election.
I suppose based on that, it's possible that this one may belong to my 'random nonsense' category rather than the 'intentional lie' one - possibly he vaguely heard something about a million voters in California and decided to interpret it as a million illegal votes without checking anything any further. Which would be merely lame and foolish from a regular Joe, but it's extremely worrisome from the president of the United States, especially considering how many devoted followers he has who do actually take him at his word.
I think the Judicial Watch story is the likelier source, considering also the timeline of when that came out and when he made his comment. But as for your theory, I'm assuming you read the sentence in your article that said 'at least one non-citizen and perhaps many more'. It's already, to put it mildly, a pretty huge leap to go from one proven case and potentially more, to 'tens of thousands', and then another pretty huge leap to go to 'one million'.
I think you'll agree that the key issue in any discussion about how many cases of voter fraud there may have been in a given election, is whether or not there might have been enough to actually swing the election. By making claims about 'one million' illegal votes or similar numbers, Trump is really suggesting that Democrats have illegally won numerous races they shouldn't have won (and no doubt trying to make the maths work for his ludicrous claims of how he'd have won the national popular vote if not for illegal votes). But the proven cases of illegal voting concern such tiny numbers that this would only be a concern for the handful of elections decided on extremely tiny margins.