I can't tell if it is part of Meltdown and Spectre. MS site says its a fix for "An elevation of privilege vulnerability exists in the Microsoft Server Message Block (SMB)".

There is a mistake in the chart. The vulerability that KB4056759 resolves is connected to CVE-2018-0749 which is not one of the Meltdown/Spectre vulnerabilities. However, CVE-2018-0749 is indeed one of the vulnerabilities, fixed by the current Security Update.

Source
An elevation of privilege vulnerability exists in the Microsoft Server
Message Block (SMB) server when an attacker who has valid credentials
attempts to open a specially crafted file over the SMB protocol on the
same machine. An attacker who successfully exploits this vulnerability
could bypass certain security checks in the operating system.
The update addresses the vulnerability by correcting how Windows SMB
Server handles such specially crafted files.
To learn more about the vulnerability, go to CVE-2018-0749.
The reason people are mistakenly making a connection, to the Meltdown/Spectre patch, is because (CVE-2017-5753, CVE-2017-5715, and CVE-2017-5754) are included in the same cumulative patch that includes KB4056759. In other words, CVE-2018-0749, is included in KB4056892(Windows 10 Version 1709). There are only three vulnerabilities associated with Meltdown/Spectre anyways.