Question about a "coup" of the Lisitsin Gambit

Hi everyone (sorry for any english mistake, i'm not a native speaker),

I'm studying the reti opening right now, and I have a question about a variation.

It goes

1. Nf3 f5 (black proposes to go for a dutch defence)

2. e4 (white decides he want a Lisitsin Gambit) fxe4

3. Ng5 d5

4. d3 Qd6 (recommended by stockfish 11 depth = 20)

My problem is with Qd6. I don't really understand the idea. I tried to think about it and I have two hypothesis :

1) it may be to prevent 5. Bf4, but it do not seems me to make so much sense...

2) it may be to prepare castling queen side, but I don't see why Qd6 would be better than Nc6 for instance, so it doesnt make much sense too.

Do anyone have any idea ? I would be very happy to read your thought !

Thanks for reading !

What An onteresting question, and don't seem to find an explantion either ; anyone ?

I'm not too familiar with the Lisistyn. I much prefer the Delayed Lisitsyn (where you play d3 before e4) and with Black when I play the Dutch I'm more inclined to just give back the pawn by playing e5 instead of d5 but I do see two ideas behind 4. ... Qd6

1. Black wants to be able to take back on e4 with their pawn without allowing Qxd8+ (which will at the very least destroy their castling rights. It's even worse if White's knight is still on g5 and can head for f7).
2. On d6 the queen supports the g6 square so if the game continues with 5. dxe4, Black can play 5. ... h6 and if now 6. Qh5+ then Black can respond with 6. ... g6.

4...Qd6 to hold the pawn is a good move, but giving back the pawn 4...e3 or 4...Nf6 also merit consideration.
The point is 4...Qd6 5 dxe4 h6 6 Qh5+ g6, when Qd6 defends pawn g6.
That is why white can better play 4...Qd6 5 Nc3.