That picture is not from Mariupol. It's in Irpin next to Kiev. And I have seen it before in the context of civilians using the bridge to hide from shelling. There's a video of it too from some reporter in a language I don't understand: https://twitter.com/La_SER/status/1500053681273724931
This does not look like civilians being held hostage at all.

As for the telegram video, it's hard to verify. It's not terribly hard to let some random woman say something in front of a camera. And looking at the rest of the channel, it seems to be an extremely pro-invasion channel as well. I guess we will have to wait until there are more details, but I highly doubt nobody was informed of the corridor.

And let's not forget the most important part: If Russia simply didn't invade, all of these humanitarian crises wouldn't have existed in the first place

https://www.pbs.org/newshour/show/russian-attacks-expand-in-ukraine-worsening-civilian-suffering

Here are what Mariupol residents are saying