I will address both the narrow and broad issues in your comment.
On the narrower issue of this specific game - I think the designers, German, did not quite appreciate how colonialism is viewed in other countries - such as the USA and UK. This is possibly because the Nazi legacy of Germany far far overshadows its colonial legacy. Needless to say, such a theme would fall at the first hurdle in the UK or in the USA. So despite what is says on the box this game really glosses over some horrid history and is not "leftist" as you say.
The broader issue, I think, is the familiar argument that Western countries get too much stick for their expansionary foreign policy in the 19th century
"expansionary foreign policy", by the way, is a diplomatic phrase. The "leftists", as you name them, would prefer the term "ruthless exploitation".
We can argue until we are blue in the face about historicism and whether we should moralise about events that took place at a time when morals were entirely different to the present day.
What is incontrovertible is that the riches reaped by our ancestors -
(well, not mine - they were Polish and under the heel of the Russian Tsars for the entire 19th century. But I identify as British and thus feel subject to the proverbial "white guilt" over the British Empire)
- will echo far into the 21st century. Albeit that British colonies seem to have done better than others.
NB. notwithstanding the brutal methods employed by the British, ref: the Mau Mau, ref: second Boer war, ref: Bengal famine, ref: opium wars, ref: Amritsar, ad infinitum.
But take for example the tragic case of the Congo. Gutted, eviscerated, disembowelled by Belgium. Is it merely a coincidence that the Congo has been for many years a chronically war torn region?
ref:
https://en.wikipedia.org/wiki/Atrocities_in_the_Congo_Free_State
The legacy of the 19th century is with us today. Our wealth is built upon the backs of the world's poor.