Sure, but if you might have to downgrade, then simply don't write code that depends on 6.5 features. 6.5 can still run 6.1 code (feature wise), so you'll be alright.
The only problem would be for people wanting to a) run 6.5, b) take advantage of newer, 6.5-only features, and THEN c) downgrade to 6.1
Ah but see that's the rub. Hindsight is always 20/20. I've actually run into similar issues in the past. Ultimately if they're consistent with versions then it's not the biggest deal (because if you have an issue with 6.5 there is probably a good chance you have an issue with 6.1 as well) but with semantic versioning I always expect code written against the major version to always work across all minor versions unless it relied on some weird bug side effect.
It just seems really inconsistent to me.
Regardless yes you should use LTS but that's no excuse for going against semantic versioning. It should be labeled an alpha or beta if they don't want to change major versions for breaking feature additions.