Memorandums are internationally treaties. The term has a different meaning in that context.

You’re right though, it doesn’t spell out any mutual defense statements.

Is a signed memorandum not binding then? Is it little more than hopes and aspirations?

The memorandum only required signatories to respect Ukrainian borders. It didn't contain any provisions for enforcement or defense if one of the signatories violated the memorandum. Since the US and UK didn't commit to do anything (beyond perhaps bringing the issue to the UN Security Council) then the question about whether the memorandum is binding of kind of moot.