I'd say high standings don't necessarily imply loyalty, but it's not a bad prima facie argument to make. Personally, I feel that standings are a social condition that exists between two parties. There's a correlation between standings and loyalty, but ultimately useful spies are held in esteem by the people they're spying on, for instance. I would suspect that the Serpentis, as an organization, don't have a lot of love for Simon, while the Federation is on the verge of giving him rights to anchor a space station. To sum up, standings say more about how a group feels about a capsuleer than how a capsuleer feels about them.
On the other hand, it's not all that hard to control standings. My corporate fire policy includes a scale for NPC's, from 'never fire on' for Federal and Republic vessels, to 'don't fire on unless it's convenient' for Mordu's Legion everywhere but Intaki Prime, to 'fire at will' for pirate faction ships everywhere and Amarr/Caldari ships in Federal space. While a pod pilot may sell his services to any faction at any time, one who almost always sells to your faction is a safer bet than one who doesn't. So, as I said above, it's a safe starting assumption that high standings likely mean some sort of loyalty, but be prepared to modify your perception as you gain information on the character in question.
That's how I do it, anyways.