One obvious difference in these expressions ... is [in] the use of the prepositions

[kadioguy]

"Sit On The Chair" Vs "Sit In The Chair". Which Is Very Correct?​

[...]

2. The intent of the speaker
In this case, the choice of expression is not determined by the chair type but by the intent of the speaker, that is, what the speaker has in mind. One obvious difference in these expressions ("sit on the chair" and "sit in the chair") is in the use of the prepositions (on and in), and these prepositions are not just there for mere fanciful reasons. When you tell someone to sit in the chair, you are invariably asking the person to make himself comfortable in the chair, which is different from when you use "sit on". That is why you sit in an armchair but sit on a bench, stool or dinning chair.

https://www.tammysenglishblog.com/2017/09/sit-on-chair-vs-sit-in-chair-which-is.html
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I wouldn't use this "in". Is it optional here?

 

[emsr2d2]

We tend to sit on chairs that don't have arms, but in chairs that do.

 

[kadioguy]

2. The intent of the speaker
[...] One obvious difference in these expressions ("sit on the chair" and "sit in the chair") is in the use of the prepositions (on and in), and these prepositions are not just there for mere fanciful reasons. [...]

Do you think that the "in" is necessary or optional?

 

[jutfrank]

The preposition gives the 'location' of the difference. The difference lies 'in' the use.

Without the preposition, the sentence is saying that the use is the difference.

I think they're two equally fine ways of expressing what the writer wants to say.