Is sealing wood with urethane or the like enough to avoid wood movement?

Will a solid wood table top still move if it is finished from all sides with polyurethane or some other water resistant finish?

Yes.

You can, in theory at least, completely seal off wood from the elements but it's really only possible using epoxy and even with that it must be applied at a substantial thickness with no pinholes or gaps in the finish.

Does that depend on temperature as well? Assuming temperature differences are big enough comparing to when wood was finished (furniture left outside for example).

No.

Wood doesn't expand and contract much with changes in temperature only (not enough to care about).

The reason temperature generally matters is that higher air temperatures often go hand in hand with higher humidity, lower temperatures with lower humidity, and it is its internal water level that makes wood change dimension.

Is that calling for trouble in case of incomplete seal or when finish gets inevitably breached by normal wear and wood starts to take up moisture again?

That would depend on if you designed the piece so that wood movement isn't allowed for :-)

It's an attractive idea, that we can seal off wood from moisture in the air to make it as stable as possible, but in reality it's a PITA to do and it's just not necessary. Only where it hasn't been allowed for is it an issue..... such as in cheap furniture built from dimensional lumber using bad plans provided free on the Internet -_- It's worth remembering that virtually everything that's ever been built from wood expands and contracts and it's not a problem because allowance has been made for it.

Where you want or need to make something that doesn't change dimension you shouldn't use wood, not solid wood at least. MDF or plywood are very good options to make something that looks like wood but is more dimensionally stable and of course there are now a wealth of synthetic materials to pick from as well.

If you need pointers on how to allow for movement see this previous Answer:
What general considerations do I need to take into account for wood movement?