The Lenovo IdeaPad Slim 5 (14", 512 GB, 16 GB, CH, Intel Core 5 210H) can generally operate two external monitors via a USB-C docking station, as long as the docking station supports DisplayPort Alternate Mode (Alt-Video) via USB-C. The HDMI port of the notebook is not required.

It is important that the notebook's USB-C port can output DisplayPort signals, which is usually the case with modern Lenovo IdeaPad Slim 5 models. With a suitable USB-C docking station that offers multiple video outputs (e.g. DisplayPort and HDMI), two monitors can be connected and operated. The docking station then also supplies power to the notebook via USB-C, so that only one cable to the dock is required.

A pure USB-C docking station without an integrated graphics card or DisplayLink technology can only control two monitors simultaneously if the notebook's USB-C port supports DisplayPort Alt-Mode with Multi-Stream Transport (MST). Otherwise, two separate video outputs are usually required for two monitors.

To summarise: Yes, with a suitable USB-C docking station that supports DisplayPort Alt-Mode, you can connect and operate two external monitors via USB-C without using the notebook's HDMI port.