...after Zelensky's office recently announced that of the 1 million people who have been mobilized, only about 300,000 have fought at the front lines. But nearly a month after his promotion, no one in the military leadership or the presidential administration has explained where those 700,000 are - or what they have been doing....

I do not equate those numbers with 700,000 casualties . A typical army, like the US military, consists of roughly 1/3 combat (front line) soldiers and around 2/3 support, and logistics troops. So, if they claim 300,000 are front line troops, then I would expect a number right around -600k - 700k to be auxiliary troops, not front line troops.

The article seems to imply that 700k soldiers are "missing," but that could simply be a miscount, or mistranslation of what was being reported. Or, they simply do not want to publicly reveal where those auxiliary troops are, or what they are doing.

TL;DR- If 1,000,000 troops are drafted, it would be normal (in most armies) to estimate that only 1/3 of those soldiers would he counted as "front line" troops.