USB 1.1 and USB 2.0 have 5V supply, ground and one pair of twisted wires. These pair must be twisted and cannot be separated to two independent wires.
USB transport complex signal including bidirectional data and commands between host and device. Each device have descriptor - some information about type of this device. PC can handle many different devices (keyboard, tablet, printer, flash drive, ...) at one port because it have have different descriptors that uniquely identifies each type of device.
Teensy can be named fake device. Because it's just a microcontroller but can emulate keyboard, mouse, or in this case modern tablet.
RS232 aka COM port have ground, TX (serial transmit), RX (serial receive) and some control wires (can be omitted in most cases). Logic one = -12V, logic zero = +12V. There are no host/device, master/slave regulations. Just connect TX from one device to RX from second and try to talk. This interface is good at long distances but relatively high voltage can kill microcontroller and digitizer.
RS232-TTL is the same interface, but voltage levels are different. Logic zero = 0V, logic one = 5V or 3.3V.
This type of interface serves the Teensy and this is required by digitizer.
So if You have Linux or Win XP, buy RS232 <-> TTL converter (mentioned at Your post) or USB -> RS232 TTL converter (mentioned in My post), search for drivers and start playing with tablet.
If You have newer Win version, You can still do that, but that's not trivial and not plug'n'play process. I cannot help here, because i tried this and finished without success.
Btw, for COM port You need to add power supply or steal 5V from USB/PS2/gameport/LPT.
WaxBee is the only way for very old tablets with ADB connector. It works well for serial tablets too.
How to use it? If You buy Teensy 2.0 (Atmega32U4 with special bootloader), download waxbee-0.16.zip from this site: https://github.com/popbee/waxbee/releases
This software allow You to create firmware and upload it into Teensy. If You pick wrong options, You can try again.
If You have clone with Atmega32U4 and different bootloader, You need to generate firmware in WaxBee, then play with AvrDude or similar software. Cheaper but more fun.