Equally then, the nuclear disasters shouldn’t count, right?

Deaths from an accident at an active nuclear power plant are not the same as deaths caused by a burst dam that was originally intended to produce electricity one day, but has never produced any. Especially if you call the statistic 'Deaths per unit of electricity production'. At the time of the accident, it was just a dam, construction of any hydroelectric facilities was nowhere near beginning, so calling it a 'hydropower accident' is highly debatable (probably as at least as debatable as calling nuclear 'conventional'). Without the inclusion of those deaths, hydro would be shown to be even safer than nuclear, given that it has produced nearly twice as much electricity in the time span covered by those statistics, while having caused a similar number of deaths (if you continue to ignore the increased miner mortality, otherwise nuclear will look way worse). The article also does not cite how they determined the number of 171000 deaths, given that estimates for the Banqian dam failure range between 26000 and 240000. The author mentions (but does not cite) a paper by Benjamin Sovacool from 2016, which analyzes the deaths caused by different forms of energy but, crucially, omits the Banqian dam death toll. I will try to get hold of that paper to see the reasoning, but I suspect it may align with mine.

How do you assume it’s ignoring their increased mortality?

The article makes zero mention of any such thing, and the section about how the deaths are calculated (footnote 3 in this section) only calls out the deaths from Chernobyl and Fukushima. Direct quote from the footnote:

Nuclear = I have calculated these figures based on the assumption of 433 deaths from Chernobyl and 2,314 from Fukushima. These figures are based on the most recent estimates from UNSCEAR and the Government of Japan. In a related article, I detail where these figures come from.

No mention at all of any other deaths or causes of death, nothing whatsoever. It's the deaths from two nuclear accidents, that's all. The figures from the cited study alone would multiply the number of nuclear deaths in this statistic. What's worse, the author has published another article on nuclear energy which essentially comes to the exact same conclusions. But if you include deaths from a burst dam that has never produced electricity (but was planned to do so eventually), then you must include deaths among people who mine the material destined to produce electricity in a nuclear plant.

To me it simply looks like the author of this article is highly biased towards nuclear, and has done very selective homework.