He said Trump GOT the letter on July 19th. You realize that snail mail takes a few days for delivery, right? Even Trump says in the clip that it must have been one of the last things JFK Jr. did.
Start at 37:00. Trump GOT the letter July 19. It does not state it was dated July 19.
https://www.justice.gov/opa/pr/former-president-maryland-based-transportation-company-indicted-11-counts-related-foreign
I don't think it's that big a deal. Strzok (and the others involved in the plan) knew McCabe was going to call Flynn that day. In his statement, McCabe says that before he could place the call, Flynn called him. While he was on the phone, he went ahead with setting up the interview. Strzok just wasn't aware that Flynn called before McCabe could place his intended call. No biggie, imo.