fwiw I've had success with the minimalist two-diode type circuit that you linked to. The only difference being that I used the TX pin of the 'sniffer' device as a handy source of -12V to use through a 47K pull-down to make sure the sniffer's own RX didn't float when neither of the snooped-upon devices was transmitting.
Unless you're driving tens of feet of wire, or are running at faster rates like 115.2kbps, the diode & resistor thing shouldn't affect the circuit too much.
If you really want to buffer the signals to TTL and back, there are of course the MAX chips, and even simple old line-driver/line-receiver type chips like the 1488 quad driver and 1489 quad receiver that would do the job.
I don't know if there is a formal definition. But consider this:
An RS232 character is about 10 symbols long (start, 8 bits, stop ).
If you start decoding halfway through a symbol, and your clock is 5% off, after 10 symbols you will have drifted off target by 50% of a symbol width; i.e. 5% is the maximum possible tolerable error. And that assumes the other end is clocked correctly; if it had an error in the other direction, you would decode it incorrectly.
So a 2.5% error in opposite directions at each end would also be on the edge of failing.
Signal integrity problems caused by cable length will only make this worse.
So practically, an error below 5% will work with a "good" clock at the other end and a very short cable; for robust communications you want a clock error below 2%, and better than that to support long cables.
Now find out what your ATTiny is capable of, at different baud rates and CPU clocks...
Best Answer
The RS-232 standard specifies the range as:
In addition, inputs must tolerate voltages up to ±25v, and outputs must tolerate indefinite shorts to ground.
In the past, a lot of equipment used ±12v since it was available from a minicomputer power supply for example. Once personal computers became popular, most of them switched to ±5 since it is commonly available and still within the spec. (Well, the +5 is commonly available, additional circuitry is necessary to get the -5V if the main power supply does not provide it.)
As an example, the MAXIM series of UART RS-232 interface chips such as the MAX220-MAX249 series use a voltage doubler and voltage converter to generate ±10v.)