...and there is no mention of using AREF. In fact, I can't even find a AREF pin.
I guess by now you're aware that the AREF pin on a tiny45 DIP chip is physical pin 5, but I thought I'd mention it anyway.
The datasheet for the tiny45 definitely says that there are two internal analogue reference voltages, 1.1V and 2.56V, so you're on the right track:-
Internal reference voltages of nominally 1.1V / 2.56V are provided on-chip. Alternatively, VCC can be used as reference
voltage for single ended channels. There is also an option to use an external voltage reference and turn-off
the internal voltage reference.
Using DrAzzy's cores, both of these compile:-
analogReference(INTERNAL1V1);
analogReference(INTERNAL2V56);
Whereas on a board without a 2.56V reference like an UNO, I get a compile error.
That's got to be an indication that it will probably work OK using either of the internal references, set as shown.
I don't know whose core you're using, but perhaps you could contact them and ask?
(Or do a test. Set the internal reference to 2.56V with 'analogReference()', then apply a known voltage to the ADC input pin and have the chip light an LED if the analogue value is within the expected range.)