I think the confusion about “feudalism is rent” stems from a lack of understanding about what capitalism is and what feudalism is. Feudalism is a very different mode of production: serfs were tied to their lords, but lords also had duties towards their serfs. This essay talks about what changed from feudalism to capitalism, which is something everyone in the 19th century was very aware of. Today, feudalism is so long ago that we have a hard time even conceiving of something that isn’t capitalism. (And so we come up with bizarre, indefensible definitions like “feudalism is when rent.”)

Marx lays out in Capital that rent, profits and interest are all crystallized surplus-value, i.e., all of them are capital:

Capital, therefore, is not only, as Adam Smith says, the command over labour. It is essentially the command over unpaid labour. All surplus-value, whatever particular form (profit, interest, or rent) it may subsequently crystallize into, is in substance the materialization of unpaid labour. The secret of the self-expansion of capital resolves itself into having the disposal of a definite quantity of other people’s unpaid labour. (Capital Vol 1)

This point is really important to him, as he tells Engels:

The best points in my book are: 1. (this is fundamental to all understanding of the facts) the two-fold character of labour according to whether it is expressed in use-value or exchange-value, which is brought out in the very First Chapter; 2. the treatment of surplus-value regardless of its particular forms as profit, interest, ground rent, etc. This will be made clear in the second volume especially.

Something novel about Marx’s books is that they look at capital from labour’s perspective, which I wrote about here. There are scenarios in which the difference between rent and interest and profit are meaningful. But for the sake of the liberation of workers from the oppression of capital, they are all the same:

In fact, the form of interest and profit of enterprise assumed by the two parts of profit, i.e., of surplus-value, expresses no relation to labour, because this relation exists only between labour and profit, or rather the surplus-value as a sum, a whole, the unity of these two parts. The proportion in which the profit is divided, and the different legal titles by which this division is sanctioned, are based on the assumption that profit is already in existence. If, therefore, the capitalist is the owner of the capital on which he operates, he pockets the whole profit, or surplus-value. It is absolutely immaterial to the labourer whether the capitalist does this, or whether he has to pay a part of it to a third person as its legal proprietor. (Capital vol 3)