The answer seems to be that Harmony is not just sympathetic to Russia, given that it represents Latvia's Russian minority, but that it has explicit ties to United Russia. This relationship resulted in Harmony being shunned by most political parties in Latvia following their parliamentary success in 2018.
From the 5th October 2018 Reuters article:
Harmony, which currently holds 24 out of the 100 seats in parliament,
recently rebranded itself as a Western-style Social Democratic party,
saying it is committed to the European Union and NATO.
But it only ended its official cooperation agreement with Putin’s
United Russia Party last year, raising concerns its transformation is
only skin deep.
Harmony could be used by Russia to influence EU affairs, said Andis
Kudors, executive director of the Centre for East European Policy
“It (Harmony) is like a kind of Trojan horse in [the] European Union
when we speak about common European Union foreign policy in regards to
The KPV backs EU and NATO membership, but its vague election program
has left many wondering what could happen if it takes power.
The 'Centre for East European Policy Studies' is an NGO with ties to the Latvian state, Germany, and the USA. Whose purpose is:
to contribute to the advancement of the Latvian foreign policy by
building-up the expertise on Russia’s development and its different
foreign policy expressions in the neighbourhood.
The BBC is reporting Harmony as being 'pro-Russian' because of Harmony's representation of the country's Russian minority and historic ties to Putin's United Russia party. To be clear, it appears that the BBC is reporting this assessment instead of inventing it, as many Latvians seem to have that opinion.