Yep. systemd-based distributions rarely use ntpd, although you can use it instead of systemd.timesyncd if you prerfer. That's what I had set up myself, but it turned out that ntpd was much slower to update time than timesyncd for some reason, so I eventually resorted to timesyncd.
Also to answer the OP, not all RTCs on motherboards are born equal. 10 seconds per day of RTC drift is about 115 ppm, which is about average (not good) for a *basic* crystal-based oscillator, and proof in itself that it's not network-synced.
But 0.3 s of drift over 15 days would be 0.23 ppm, which, while not completely impossible with a good TCXO, is completely unlikely to be what you get with any consumer PC RTC, so it's evidently network-synced.