#hoshireno

As for the latter: in canon they have a good vibe: modest/sort of subtle but friendly chemistry:

mutually in ch 22.

Hoshina comforting Reno about his "low" RCP in ch 5, admiring Reno's progress in a friendly way in e.g. ch 11, etc.

Reno clearly isn't meh about the kind of person Hoshina is (wouldn't have a reason to anyway: Hoshina is not high-handed, aggressive, haughty, etc, unlike e.g. Shinomiya, whom Reno dislikes: see section "a specific aspect of his social dynamics" in Reno in the manga vs in other kn8 official content). And Hoshina seems maybe even fond of Reno. Not that he easily dislikes people anyway, it seems, so it's not unique, but still: friendly chemistry. So they seem to like each other at least in the basic sense. And based on their personalities, they likely do or would get along pretty well.

As for the former & why it feels more intimate than in canon:

Reno does have a spine, can be blunt af with strangers (see his first interaction with Kafka in ch 1, below), and sure would make an observation like that (again: see e.g. ch 11, 31):

again: dry wit. Likely thx to his seemingly innate high ability for rational thinking, e.g. even as a kid he processed the tragedies he witnessed as "nobody came to help", not as "I should have been able to help".

but also sincerity/directness (e.g. his behavior in even just ch 1; it's also stated by Iharu in ch 62 (see below), etc) and low-drama. See also the upcoming post I'll have (likely "Reno is so real").

But his style seems plain honest, he's not a cheeky/playful type. E.g:

his response to Iharu's boasting in ch 11 (see above). They're close friends there already (as seen in the jap original & eng translations that keep honorifics, Reno is using Iharu's given name with "-kun" = an address term for close male friends) but Iharu is being a bit out there. But Reno's response is "a bit sarcastic yet low-drama" and all about what's true, not about just biting back etc. Dry wit. That's his style. Btw: see for a translation analysis of that scene.

his subtle roast of Shinomiya's — whom he dislikes, see section "a specific aspect of his social dynamics" in Reno in the manga vs in other kn8 official content — whole personality in ch 26 (see below) + in a panel by Matsumoto from the light novel (see below): plain dry wit. Not a friendly jab, he's looking unimpressed, his wording is rather annoyed, like a "all that axe makes me think of is the brutal personality I have to endure".

He can go a bit playful, if he's very close/comfortable with somebody — e.g. his tease towards Kafka in ch 31 (see above), he likely teases Iharu like that sometimes too. But he's not implied to be as close with Hoshina. Moreover, even in the aforementioned moments he still never has the kind of teasing/playful vibe like in "Hoshina's Day Off" (whether the anime or the manga version), just that matter-of-factness/dry wit of his. It's subtle but makes a difference. See also on how & why Reno addresses others.

So, that ep:

gives Reno a "closer than in canon" vibe with Hoshina.

but also gets Reno's style just a bit off. In that particular moment among others, and for other characters as well. Which is for whatever reason common (likely some of it is plain commercial appeal: making the dynamics more engaging, changing the original portrayals to be more fun or pleasant).

As for the former though, again, it's believable — in canon he seems to like Hoshina & vice versa — unlike e.g. the more or less friendliness that his canonical dislike of Shinomiya is swapped for in e.g. "relax", the anime, etc. Speaking of...

...it's weird that despite:

his good canon chemistry with Hoshina.

both Reno and especially Hoshina being among the more popular characters.