It’s a national team, therefore they’re representing their countries in an official capacity, regardless of their personal stances. It’s more akin to Nazi Germany sending a team to the Olympic games and the controversy that surrounded that. That’s the unfortunate nature of flags and sports, but it’s how it is.

Edit: It appears the Russian fencer was participating as a neutral participant and not as a member of a Russian sponsored team. That makes it objectively unsporting on the Ukranian fencer’s behalf, however subjectively I at least understand the problem. Just because the Russian fencer isn’t participating as part of their national team (because their team was not allowed to participate) doesn’t mean they aren’t otherwise that team’s fencer and are still, unofficially, representing their country.