Well, those are the only studies I can find on the topic.
The only situation in which I’d be concerned that precum contained sperm is if you had very recently ejaculated and hadn’t pissed afterwards. In that case it seems plausible that some sperm may be left in the urethra and may “hitch a ride” in the next fluid to exit (precum). The number would be pretty darn low (in the “infertile” range), but pregnancy could still potentially occur.
In any case, I’m paranoid about pregnancy, so I always combine two methods: (1) pill (or other hormonal method) & condom, (2) pill & withdrawal (I know my timing exactly and have never messed that up), or (3) condom & withdrawal (I’ve had plenty of condom mishaps, so I always pull out still).