Jeff is all sorts of things I don't care for, but he's an able lawyer and he understands the purpose of evasion as well as anyone. His omission was just that and it was intentional. He knew he'd met with the Russians on multiple occasions and he knew that he should disclose that and could have if he hadn't minded the heat to come with, "No, though I met with the Russian ambassador in my capacity as a senator, unrelated to the campaign in any particular and on a couple of occasions." Hopefully, he could have added, "I've done that any number of times over the course of my service to the Senate. This wasn't remarkable or related and I offer it purely in the interest of full disclosure."
What's understandable in the average person, his, "In retrospect I should have slowed down," as if he didn't think about it, rushed through it, isn't acceptable from an attorney with numbers of depositions, cross examinations, etc.
That said, absent hard evidence that he did something untoward or unethical I don't see this as justifying calls for him to step down. The recusal is reasonable enough. He made a poor choice. We can't know why, but it doesn't rise to the level of perjury as I read it.