The use of the term 'substitute' is referring to the fact that although the initial candidacy filing period of October 1st to October 8th has expired, Sara Duterte-Carpio (or another candidate) could still run for president by entering the race as a substitute candidate pursuant to section 77 of the Omnibus Election Code.
Sec. 77. Candidates in case of death, disqualification or withdrawal
If after the last day for the filing of certificates of
candidacy, an official candidate of a registered or accredited
political party dies, withdraws or is disqualified for any cause, only
a person belonging to, and certified by, the same political party may
file a certificate of candidacy to replace the candidate who died,
withdrew or was disqualified. The substitute candidate nominated by
the political party concerned may file his certificate of candidacy
for the office affected in accordance with the preceding sections not
later than mid-day of the day of the election. If the death,
withdrawal or disqualification should occur between the day before the
election and mid-day of election day, said certificate may be filed
with any board of election inspectors in the political subdivision
where he is a candidate, or, in the case of candidates to be voted for
by the entire electorate of the country, with the Commission.
According to Resolution 10965 from the Phillippines Commission on Elections, in the case of the 2022 elections, political parties have until November 15th to name a substitute candidate to replace their party's initial presidential candidate. This can be for three reasons:
After November 15th, a candidate may only be substituted for the first two reasons, up until midday on election day (May 9th 2022).
It was for this third reason that President Duterte was elected in 2016; PDP-Laban had initially named Martin Diño as their candidate for president, but he withdrew at the end of October, naming Duterte as his substitute. Duterte officially filed his certificate of candidacy at the end of November 2015 and was subsequently successful in the election.
The substitute, therefore, would most likely be the current PDP-Laban candidate, Senator Ronald "Bato" Dela Rosa, and indeed, Dela Rosa has indicated that he might be willing to step down should Duterte-Carpio decide to run:
“As I have said, mahirap man magkunwari pero we are still awaiting the
final decision. Kung talagang gustong tumakbo si Inday Sara, I am
open, I am willing to give [way] to her,” Dela Rosa added.
(As I have said, it is hard to pretend but we are still awaiting the
final decision. If Inday Sara wants to run for president, I am open
and willing to give way to her.)