I would like to buy Arduino MKR WiFi 1010 to utilize the web IDE that Arduino provides. I am just worried that it might not be supported by Sparkfun Simultaneous RFID reader library.
The hardware isn't supported so you don't have to care for the library. The schematics define clearly: "VCC must be 3.7-5.5V". The serial signals are converted to that VCC level so you cannot connect to any MKR series Arduino (or any other 3.3V device).