Esto is an indicative and determinant, but it doesn't determine the genre of the noun. If you see the sentence you see that Esto indicates and then the author uses UNA, that determines the sex.

Because when you use "Esto" you refer to the table as an object, (object = male), but you can also use "esta"

"Esto" in this instance means "this one" so, it's a noun rather than an adjective having to agree with mesa. A masculine "this" would be "este" rather than "esto".

"Esto" is a neutral pronoun, it can be used instead of "éste" (male) or "ésta" (female). Notice the difference, demostrative pronouns have accent while demostrative adjectives don't. For example: Esta mesa (adjective) "Ésta es una mesa" or "Esto es una mesa" (pronoun)

Contrary to ALL other demonstratives, ESTO is "unspecified" for gender and number (i.e., it is neither masculine, nor feminine, and neither singular nor plural), and CANNOT agree either with the nominal predicate or with the verb SER. When nouns are singular, it seems to agree in number (cf. ESTO ES UN ÁTOMO), but it is the nominal predicate (UN ÁTOMO) that really agrees with SER. [In this ESTO resembles English THERE, which is indistinctly followed by IS or ARE depending on the number of the complement of BE]. Since ESTO lacks agreement features, it cannot be used as a demonstrative adjective: adjectives must agree, and Spanish nouns are masculine or feminine, but never unspecified (and we no longer have "neuters"), so, NO Spanish noun can immediately follow ESTO (cf. *esto coche, *esto célula, *esto tomates, etc.). Nouns may be gender-invariable (e.g., un/a atleta), but in that case we use estE/estA, as the case may be, but never *estO (cf. este/a atleta, not *esto atleta). ESTO MUST be used "pronominally", but it differs from all other demonstrative pronouns, which have agreement and MUST agree with the noun predicate (cf. ÉSTA es LA MESA; ÉSTE es EL COMEDOR; ÉSTOS son MIS HIJOS; ÉSTAS son mis HIJAS, etc.). On the contrary, ESTO does not, observe: ESTO es un/el CABLE (= masculine, countable, singular) ESTO es una/la LÁMPARA (= feminine, countable, singular) ESTO es CAFÉ (= masculine, singular, non-countable) ESTO es HARINA (= feminine, singular, non-countable) ESTO es LO que me molesta (= neuter, singular, non-countable) ESTO SON residuOS nucleares (= masculine, countable, plural) ESTO SON célulAS cancerosas (= feminine, countable, plural) ¿Qué es ESTO? -> (ESTO) Es un/una.... Son residuos/células It is its lack of specifications that makes ESTO so versatile, like German DAS, cf. Das ist ein Wagen/eine Lampe/ein Mädchen; Das sind Wagen/Lampen/Mädchen; Was ist DAS?), French CE, etc.