Sounds fine for 2.5 season: https://splodzblogz.co.uk/2018/11/05/review-oex-fathom/
Yes, it is a big thing even packed. So you are probably correct. But for less than £40 delivered i think it's a decent bag :)
Would trust the Marmot ratings far more though. The key thing to notice is that, while the Marmot isn't much more expensive than the RRP for the OP, the Marmot is 300g heavier, which makes the ratings way more believable.
I own a Marmot Trestles 30 with similar rating (Comfort rating: 2 °C Lower limit rating: -3 °C Extreme rating: -19 °C) and I have never had an issue in a Scottish bothy any time of the year...
What are these like when its not so cold?