"The ubiquity of zero-commission and fractional-share trading means that diversification and customization is easier than ever"
I think what they are relying on is the fact that nowadays you can own fractional shares (so you don't have to spend that much money to own an index since you no longer care about owning whole number units) as well as zero cost trading (so you don't really have costs to maintaining a portfolio or creating one).
Note that in most cases the bid-ask spreads are low (since for major indices there is enough liquidity in their constituents), and as such won't meaningfully contribute to your gains / losses unless you are rebalancing often enough to factor this in(in which case, making an index is pointless).
I think what they are relying on is the fact that nowadays you can own fractional shares (so you don't have to spend that much money to own an index since you no longer care about owning whole number units) as well as zero cost trading (so you don't really have costs to maintaining a portfolio or creating one).
Note that in most cases the bid-ask spreads are low (since for major indices there is enough liquidity in their constituents), and as such won't meaningfully contribute to your gains / losses unless you are rebalancing often enough to factor this in(in which case, making an index is pointless).