The ITUC actually mentioned the 2010 Labor Code and trade unions in another report (page 2):

In 2010, Kuwait enacted a new Labour Code but restrictions on trade union rights remain significant. The right to form or join a union is granted to Kuwaiti nationals but foreign workers are only allowed to join a union; they are barred from electing officers or being elected. Additionally, migrant workers need a certificate of good conduct and moral standing. The law also stipulates that every union needs at least 15 national Kuwaiti members and 100 members in total in order to be certified by the state. This condition affects organising for 80 per cent of the country's workforce. Union membership is further weakened by the exclusion of domestic workers from the Labour Code. The Maritime Act or of the Oil Sector Labour Law also set special rules on unionisation in these two sectors.

If that isn't enough, there are many other sources that confirm foreign workers can join unions. You can also google it in Arabic.

This is an Overseas Business Risk report by the Foreign and Commonwealth Office, it was updated on May 9 2016:

Whilst Kuwaitis benefit from generous public subsidy (which in recent months has seen threats of dwindling because of falling oil prices), are unionised and have the right to strike, this right is not fully extended to expatriate workers – who comprise the majority of the labour force in Kuwait, but can only become non-voting union members after working for five years.