As Nicolas mentioned, concat is a top-level function and doesn't have an equivalent pd.DataFrame method. Other than checking the documentation, you can check if there's a concat method by:

import pandas as pd
hasattr(pd.DataFrame, 'concat')   # False
hasattr(pd, 'concat')             # True

The following are the list of top-level functions that don't have an equivalent pd.DataFrame method:

from inspect import getmembers, isfunction
{n for n,_ in getmembers(pd, isfunction)} - set(dir(pd.DataFrame)) - set(dir(pd.Series))

• bdate_range, date_range, interval_range, period_range, timedelta_range
• concat
• crosstab
• cut, qcut
• get_dummies
• infer_freq
• json_normalize
• lreshape
• merge_asof, merge_ordered
• read_clipboard, read_csv, read_excel, read_feather, read_fwf, read_gbq, read_hdf, read_html, read_json, read_orc, read_parquet, read_pickle, read_sas, read_spss, read_sql, read_sql_query, read_sql_table, read_stata, read_table, read_xml
• set_eng_float_format
• show_versions
• test
• to_datetime, to_numeric, to_timedelta
• wide_to_long