What did "make love" mean in the 1920s?

Solution 1:

From the early 1500's until perhaps the mid-1900's, the following sense of make love was common:

(now archaic) To make amorous approaches to; to woo, romance, court.

To illustrate this sense, wiktionary provides five quotations, dated from 1501 through 1946. For example:

1815, Jane Austen, Emma:
scarcely had she begun, scarcely had they passed the sweep-gate and joined the other carriage, than she found her subject cut up – her hand seized – her attention demanded, and Mr. Elton actually making violent love to her: availing himself of the precious opportunity, declaring sentiments which must already be well known [...].

In the referenced scene, Emma and Mr. Elton are riding in a closed carriage after a Christmas entertainment; Mr. Elton takes hold of Emma's hand and begins telling her of his “ardent attachment” and “unequalled love” for her. That is, he is making amorous advances, wooing, romancing, courting; and no more than that.

The advice mentioned in the question – to “Make love to your wife; continue to be her sweetheart” – does not suggest sex per se, but instead suggests wooing, romancing, courting.

Solution 2:

Per @medica's comment, a good dictionary should help you. So here's the relevant bit from OED...

to make love (under love noun, Phrase sense 3 b)
orig. U.S. To engage in sexual intercourse, esp. considered as an act of love. Freq. with to, with.

...for which their first citation is...

1927 J. S. Bolan Deposition in L. Schlissel 3 Plays Mae West (1997) 218
Jimmy embraces Margie LaMont and goes through with her the business of making love to her by lying on top of her on a couch, each embracing the other. 1

Realistically, there's no chance that the writer of OP's 1921 citation would have expected readers to understand make love = have sexual intercourse. He just meant be loving and attentive, as you were when you were courting.

1 There's some discussion here about that "first citation" (i.e. - Why is it both 1927 and 1997?, and How do we know they weren't just cuddling/snogging?)