Where in GME's 2019 public filings are there short-stock positions reported?
Nowhere. They are NOT PART OF GME's PUBLIC FILINGS. Why? The are not part of the finances or anything related to the GME internals. It is OTHERS that are borrowing the stock and selling it (a.k.a. short), the company has zero information about it. Why would it?
and how can we know that the company was "heavily" shorted?
By looking at the records for those. Various sources are publishing the short numbers of stocks, i.e. the Wall Street Journal or Bloomberg.
Again, the short sale is not an attribute of the company at all, so expecting it in the filings shows a certain naiveté about what the filings contain and how the stock market works.
This limited knowledge is also shown in your statement: "GameStop's stock is someone noticed that it has strong finances" - in what world has a company that has a losing business model and is losing cash every year without turning it around a company with "strong finances"? This is the type of company going straight into a bankruptcy - hence a lot of professionals opened (obviously too many) shorts. They have been losing money 2 years straight, no income the year before as per their EPS. The business model is outdated (online retailers and ebay likely earning their profit) with the revenue going REALLY down - 30% less turnover in 3 years is NOT growing. Want to see strong financials? Look at Microsoft, blowing out the profit targets big time. Not a lot of people shorting into a company with growing profits quarter after quarter after quarter for years. THOSE are strong financials - GameStop is a takeover / bankruptcy candidate, and no one wants to buy their business model.