yes it is - because the person you're buying it from now can invest the proceeds of that sale (to something else).
Buying shares off the sharemarket is investing - not because you're directly giving the company your money, but because your buying is the end of a chain of investment that directly leads to the IPO at the other end. Same with housing.
Your way of seeing only direct investment as "true" investment is too shallow, and lacks the actual nuance of how the financial markets operate. If you sell your shares to somebody else after an IPO, the IPO might not ever have happened, or that the raising of money costs more (to account for the risk of having to hold the bag till the company profits). By strata-ing the risk out to different people, the financial market allows all levels of risk to be bought, by all manner of different people.
The direct "investment only" method is as primitive as bartering for goods/services.
Non invested savings are zero sum. This is just macro economics. If one person has a savings surplus, the other person has a savings deficit (unemployment). Buying existing housing isn't an investment. Building new housing is. Buying an existing company isn't an investment. Starting new companies or growing old companies is.
Non invested savings are growing, just look at treasury bond yields and interest rates. If you buy $100 worth of treasury bonds the government has to issue $100 in debt, otherwise yields drop. If the government doesn't issue additional debt it cannot invest the additional money on behalf of buyers of the treasury bonds. Conventionally, inflation would rise immediately because more money is circulating in the economy but it doesn't, implying excess savings that are not being invested.