But the standard Apache server returns 200 OK for http://example.com/page.htm/garbage/more-garbage/even-more-garbage/big-garbage-collection/only-garbage
Yes, it does, and serves the requested page that ends at .htm, ignoring the trailing garbage.
I've been hit by this, by a bot that says it's Baidu spider. Its IP traces to Baidu, but it is very badly behaved, doesn't seem to be working in a coordinated way with the other legitimate Baidu crawlers, and it consumes gigabytes of bandwidth. Like yours, mine mixes and matches directories that really do exist, in bizarre random combinations. I've thought this might be either a hacked crawling computer at the company, or one whose programming has gone horribly wrong, but there might be other possible explanations. It's been going on since at least early October.
My pages are static, too.
Does the IP you see actually trace to Google, Inc.?
I believe your countermeasure would more properly be: