No, the SMSes synced aren’t searchable by content. In fact, the SMSes are searchable by the numbers. Just type the number in the search tab and you’ll get the SMSes from that particular number.
Searching SMSes based on content creates confusion. Different texts you receive or send may have some word in common but there might not be any logical connection between them. Therefore, we deliberately removed the SMS text search after feedbacks in the beta version of the App.