The tester can't tell the difference. One way to look at is that electrically, since neutral and ground are already bonded at one location (normally the main panel), the electrons don't know the difference. But the other way to think about it is to look at what the tester is actually doing.
What do the Lights Indicate?
These testers are actually 3 simple lights connected as follows (left to right on your particular tester, others may vary):
Hot to Ground
This will light up provided that Hot is indeed Hot (and not Neutral) and Ground is functional.
Note that Ground being functional in this context does not mean "connected to a water pipe or rebar or a ground rod", though that alone might be enough. It means "ground connected to neutral in the main panel".
That also means that if Ground is connected directly to neutral in this receptacle box, the tester does not see that as a problem.
Hot to Neutral
This will light up provided that Hot is indeed Hot (and not Neutral) and Neutral is functional.
Neutral to Ground
Since Neutral and Ground are bonded in the main panel, this light is normally off. Connecting them inside the receptacle box won't make any difference. Where this does light up is if something else is really messed up so that current flows between neutral and ground. The most likely scenarios are Hot swapped with either Neutral or Ground, so the tester "identifies" those conditions but those are really not guarantees as there are other possible situations.
What do the Light Combinations Mean?
There are 8 possible combinations. But "all on" would be really strange (but possible if someone wires a 120V receptacle to a 240V breaker) so that is not included. The other combination not included is Off, Off, On. That would mean current flowing only between Ground and Neutral (the exact opposite of this question), and no current flowing between Hot & Neutral or between Hot & Ground, which would be pretty hard to do (Open Hot + Hot/Neutral swapped?)
Just remember, the titles assigned are based on the most common scenarios but are not the only possible problems.
Off, On, Off = Open Ground - Hot to Neutral but no Hot to Ground
On, Off, Off = Open Neutral - Hot to Ground but no Hot to Neutral
Off, Off, Off = Open Hot - But really means "nothing at all" - e.g., could be "Open Ground + Open Neutral but Hot OK" - so don't assume this means there is no hot wire present - that could be a deadly mistake!
On, Off, On = Hot/Ground Reverse - Hot to Ground OK, but not Hot to Neutral, so the likely situation is that the Hot & Ground wires are reversed but it could be something else.
Off, On, On = Hot/Neutral Reverse - Hot to Neutral OK, but not Hot to Ground, so the likely situation is that the Hot & Neutral wires are reversed but it could be something else.
On, On, Off = Correct - Except, it really only tells you that Hot is in the right place and that all Ground & Neutral are, collectively, correct. It does not tell you, for example, if Ground & Neutral are reversed or, as originally asked, if Ground & Neutral are combined.
Could Such a Tester Be Made?
The one possibility I can think of is based on any wire (ignoring superconductors - we are talking about normal residential copper wires) having some resistance. So a "normal" neutral to ground path (i.e., round-trip to main panel) would have some resistance - a quick search showing 2.525 Ohms/1,000 feet) on 14 AWG - you could measure that resistance. If you have 50' of neutral in one direction and 50' of ground in the other direction, that would be a total of 0.2525 Ohms - small but measurable, compared to essentially 0 Ohms if ground & neutral are connected in the receptacle. But this would require some pretty careful measurements that you simply aren't going to get from a $10 receptacle tester. Plus it would need even higher accuracy to work on 20' length of 12 AWG wire. Just open the receptacle and check it out...