Yes, that indicates preboot. In most Garmin devices, it's held only briefly so you need to be quick to start the flash from Updater.exe. Once the device and computer exchange data packets, it will retain preboot mode until the flash finishes or is otherwise interrupted. Follow this:
  • Load 102001000260.rgn into Updater.exe and select the radio button next to "USB Device", keep the GUI visible on your desktop with cursor positioned over "OK" button
  • Have a known-good USB cable directly connected to a USB2 port (avoid USB3 or a hub)
  • Start the up+power combination for preboot and connect the cable to the foretrex
  • Watch the window next to "USB Device" on Updater's GUI for it to be populated (depending on settings you might hear the Windows device connect sound too)
  • IMMEDIATELY you see the device details appear either click OK or hit Enter on your keyboard

The flash should start. If it doesn't, repeat the preboot process and try to be quicker. As you saw with it's brief appearance in Device Manager list it only shows for a matter of seconds before disconnecting. If the flash starts but doesn't complete with Updater reporting a error message, then keep trying. Sometimes devices that have been previously wrongly-flashed are difficult to re-flash correctly but persistent multiple attempts will eventually be successful.