That’s not true in general. Plenty of jurisdictions do not allow you to use contradictory arguments in hopes that one is persuasive.

That sounds bad to me!

The TOS argument was so awful it deserves some kind of bonus loss, but let's put that aside for now.

Let's say I paint a fence, and my neighbor complains I ruined a particular segment that shouldn't have been painted. Why can't I argue that this segment was on my property and that my neighbor gave permission to use that paint on all segments on their property?

Let's say there's a 15% chance the survey was wrong about the fence location, and a 15% chance the court disagrees with how I'm interpreting the agreement. If I can use both arguments I have a 2% chance of losing, which seems fair to me. Both arguments are reasonable, so why should I have to risk it on only one?

In the context of this story (ie. US), it's specifically allowed: https://en.wikipedia.org/wiki/Alternative_pleading#Civil_law