It isn't socialist to lower taxes, and it isn't socialist to raise them either - at least not inherently.
To understand why, it's necessary to discuss what socialism actually is.
Socialism is an economic philosophy which advocates for social ownership of the means of production, as opposed to private ownership.
State-oriented conceptions of socialism, including reformist ideas such as social democracy, typically conceive of state ownership as public - with the state being considered a proxy for the people.
To that end, socialist states may increase taxation to fund state-owned industries or services. As states which characterise themselves as socialist typically undertake a greater amount of activity, taxation increases have become associated with left-wing government.
However, in that case, it isn't the increase in taxation itself which is socialist - it is the system which the increase in taxation supports. It is possible to increase taxation for reasons which are not socialist, because socialism is not simply when the government does things.
There is an (often intentional) misinterpretation of socialism which stipulates that it is when "free stuff" is given out, whether or not there are enough resources to sustain this behaviour.
Sunak is weaponising this misinterpretation, and leveraging it against Truss.
In the clip, Sunak implies that Truss' proposed tax cuts constitute "free stuff". They're a policy which he claims to be something-for-nothing - to be unsustainable, due to his belief that they will fuel inflationary pressures.
Therefore, through the lens of the misinterpretation, the tax cuts can be construed as socialist. However, as we understand the actual properties and philosophy of socialism, we can determine that they are not - because they don't intend to advance social ownership.
Sunak chose to leverage this misinterpretation because "socialist" is effectively used as a discrediting insult amongst conservatives. Shortly before, Sunak had also been accused of "socialist policy" for tax increases which he had pursued as Chancellor - an accusation which is similarly ill-founded.