It's because like is not being compared with like. The tested and confirmed flu deaths are in the order of a few hundred per year, even in bad years, whereas the figures of tens of thousands of flu cases are extrapolations from excess winter deaths, where not all are tested or ascribed to flu as a cause. In general, deaths from all causes go up during a flu outbreak across a season. Just what proportion of these deaths are ascribable to flu or where flu is a contributory factor is a controversial topic but the NHS usually copes because many of these cases die at home and those that are hospitalised and die there are spread over months.
https://www.bmj.com/content/361/bmj.k2795/rr-6
It's self-evident that what has happened in China, Iran, Italy and Spain etc. is nothing like a bad flu season. The stats from Wuhan for confirmed hospitalised cases showed that in their critical category, there was a 49% mortality. For deaths overall, 33% had no co-morbid condition. Another set of earlier stats had 100% of Critical cases with bi-lobar pneumonia. The only good point from this was that all the Wuhan deaths were in the Critical category, so even the cases one grade below that, "Severe", survived.
There will be excess winter deaths due to Covid-19; just that we don't know yet how many. While Covid-19 is very much not flu, it does cause flu-like symptoms and there is a reporting system for these. It is worth keeping an eye on the weekly reports. The most recent one showed a rise.
https://www.gov.uk/government/statistics/weekly-national-flu-reports-2019-to-2020-season
Chinese case categories:
Mild included non-pneumonia and mild pneumonia cases. Severe was characterized by dyspnea, respiratory frequency ≥30/minute, blood oxygen saturation ≤93%, PaO2/FiO2 ratio <300, and/or lung infiltrates >50% within 24–48 hours. Critical cases were those that exhibited respiratory failure, septic shock, and/or multiple organ dysfunction/failure