I think that you are wrong both in your definition of social democracy, and in your understanding of how meaningful a reform the NEP was under Lenin.

"Social democratic" is conventionally part of the name of center-left political parties in multi-party political systems in which elections are a genuine mechanism by which leadership is chosen, which adheres to policies which favor a strong welfare state, significant regulation of private sector economic activity to further the public good, and even nationalization of industries when that would prevent monopoly power or would create efficiencies and advance social good due to economy of scale.

But, a social democrat does not generally aspire to predominantly nationalize private sector economic activity, does not afford a central role to central planning of economic activity, and is anti-authoritarian.

[Lenin's NEP][1] may have allowed a limited small business private sector economy, but nationalization of all big businesses which were administered on a central planning basis, an authoritarian single party political system, and enforced loyalty to the state were all still firmly in place and as a result it would be conventional and appropriate to call Lenin a "communist" even though he experimented on a very limited basis in an economically unimportant part of the economy with alternative models.

Similarly, while the NEP "abolished *prodrazvyorstka* (forced grain requisition) and introduced *prodnalog*: a tax on farmers, payable in the form of raw agricultural product", the bottom line was that farmers were handing over lots of the grain that they produced to the government either way in a manner not all that different from the feudal system that prevailed for centuries under the tsars.

In the same sense, we don't say that the economic system that prevails in Hong Kong is not capitalist, even though at the family and extended family scale, property right break down and people work according to their abilities and receive according to their needs, and even though there are charitable activity in their civic society that also does not operate on a market basis.

The notion that "political power was given to the [average] citizen" under Lenin in the Soviet Union vastly exaggerates the extent to which the NEP changed the status quo. Mostly it changed how farming was administered, replacing large factory style communes that didn't work with more conventional farming estates, in the wake of the 1921 famine caused by overdoing agricultural reform. It had little impact in cities or in most industries, or outside of the economic sphere.

Also, keep in mind that the NEP lasted only from 1922 to 1928 (when Stalin abolished it), and was devised from the direction of a die hard communist experimenting in the direction of a less statist economic policy. It was devised and implemented from the top down, and abolished in the same way.

Conceivably, given more time and encouragement, it might have blossomed into a more free economic system, with greater political and social freedoms over time and allowed Lenin to mature from a communist into a socialist. But, it was nipped in the bud before this more or less purely economic and not social or political experimentation reached a point at which it could legitimately be called social democracy.

  [1]: https://en.wikipedia.org/wiki/New_Economic_Policy