２人っきりでデート・・もとい、ＯＦＦ会だと思ったけど、そこまで甘くもないよな。

・・、、。

I thought it'd just be the two of us on a date... I mean, an offline meetup, but I guess it wasn't going to be that sweet.

Literal

Two-people-only [with-で] date... correction, OFF-meet is [quotative-と] thought-[but-けど], to-that-extent sweet is-not-[emphatic-よ][confirmation-な].

もとい is an interjection used to correct oneself mid-sentence — 'I mean...', 'rather', 'correction'. The speaker started to say 'date' (デート) and immediately walked it back to オフ会 (offline meetup), showing they were indulging a fantasy they had to correct. 2人っきり uses the emphatic suffix ~きり ('only, nothing but'). 甘くない (literally 'not sweet') is idiomatic for 'not as easy as I thought', 'not going to go my way'. オフ会 (off-meet) is net culture for an in-person gathering of online friends.