I agree with you. Although in this match Inter is favored to beat Lazio, make no mistake, Lazio is currently performing impressively, they are currently in their best form and have high confidence after defeating Napoli in a row in the Coppa Italia and Serie A. Even though we know Napoli is currently under the guidance of Conte and has turned into a strong team, but who would have thought Lazio managed to beat Napoli.
Lazio by these wins, show that they did not simply beat Napoli one time by luck but they did it twice, and it means a lot about their quality and competitiveness. It means Lazio are strong enough to repeatedly beat Napoli, and sure they are able to beat Inter Milan. Even they are considered as weaker than Inter Milan, but they can make another surprise by beating another stronger opponent.
I understand that with Napoli win, Lazio looks like a great team but Inter isn't a simple team neither. Maybe I am biased because I always liked Inter even when I was younger, not that I am Italian or anything, just liked their colours to be fair, and I think Inter is still the better team. I know Lazio isn't a simple feat neither, Inter winning that game will of course be hard, but I do not think that it will be that impossible, I believe it will happen without much trouble and could be easier to handle.
Of course it is going to be a bigger deal when Lazio wins because that would put them close to top and Inter to get lower, but if Inter wins, I think the standings will not change that much at all, it would be pretty much the same. There is a reason why they gave 2.20 to Inter and 3.40 to Lazio, because even oddmakers are aware that it would be a very slight chance for Lazio to win, sure Inter will not dominate, it is not like they got 1.40 odds, but still, they have a big chance.